Constraint Layout performs different on different devices

Question

I'm trying to convert my relative layout into a Constraint layout to fit every device, but I'm having some problems. On the editor in Android Studio it looks like this:

I made it using a Pixel C as emulator. Now, I tried the app on a Samsung Galaxy tab and this is the result:

Why the buttons are not at center at the screen like in the first picture? This is the code of the xml, as you can see I've even anchored the element to keep the distance between them:

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/relativeLayout"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/background_main"
android:scaleType="centerCrop"
tools:context=".MainActivity">

<TextView
    android:id="@+id/textView2"
    android:layout_width="708dp"
    android:layout_height="231dp"
    android:layout_marginTop="152dp"
    android:layout_marginEnd="8dp"
    android:layout_marginBottom="8dp"
    android:fontFamily="@font/kotta_one"
    android:text="Test del pensiero\n       divergente"
    android:textColor="#000000"
    android:textSize="86dp"
    android:textStyle="bold"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.57"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.195" />

<Button
    android:id="@+id/button_1"
    android:layout_width="180dp"
    android:layout_height="60dp"
    android:layout_marginStart="8dp"
    android:layout_marginEnd="76dp"
    android:layout_marginBottom="8dp"
    android:background="@drawable/button_bg"
    android:text="@string/invioA"
    android:textAllCaps="false"
    android:textColor="#FFFFFF"
    android:textSize="18dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toStartOf="@+id/button_2"
    app:layout_constraintHorizontal_bias="1.0"
    app:layout_constraintStart_toStartOf="@+id/textView2"
    app:layout_constraintTop_toBottomOf="@+id/textView2"
    app:layout_constraintVertical_bias="0.028" />

<Button
    android:id="@+id/button_2"
    android:layout_width="180dp"
    android:layout_height="60dp"
    android:layout_marginEnd="188dp"
    android:layout_marginBottom="8dp"
    android:background="@drawable/button_bg"
    android:text="@string/invioB"
    android:textAllCaps="false"
    android:textColor="#FFFFFF"
    android:textSize="18dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="@+id/textView2"
    app:layout_constraintTop_toBottomOf="@+id/textView2"
    app:layout_constraintVertical_bias="0.028" />

<Button
    android:id="@+id/button_3"
    android:layout_width="161dp"
    android:layout_height="114dp"
    android:layout_marginEnd="8dp"
    android:background="@android:color/transparent"
    android:fontFamily="@font/kotta_one"
    android:text="Area\nDocenti"
    android:textAllCaps="false"
    android:textColor="#030000"
    android:textSize="26dp"
    app:layout_constraintEnd_toStartOf="@+id/textView2"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

Answer 1

You are giving wrong width and height to button and textview.

Also, for horizontal button and same width and height you can use chain like below I used.

Here, I give you approx design and also you can use your ConstraintLayout because I have androidx migrated project.

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/relativeLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/add_news_feed_bg"
    android:scaleType="centerCrop">

    <TextView
        android:id="@+id/textView2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:text="Test del pensiero\n       divergente"
        android:textColor="#000000"
        android:textSize="86dp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button_1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/bg_toolbar"
        android:text="invioA"
        android:textAllCaps="false"
        android:textColor="#FFFFFF"
        android:textSize="18dp"
        app:layout_constraintBottom_toBottomOf="@+id/button_2"
        app:layout_constraintEnd_toStartOf="@+id/button_2"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/button_2" />

    <Button
        android:id="@+id/button_2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:background="@color/bg_toolbar"
        android:text="invioB"
        android:textAllCaps="false"
        android:textColor="#FFFFFF"
        android:textSize="18dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/button_1"
        app:layout_constraintTop_toBottomOf="@+id/textView2" />

    <Button
        android:id="@+id/button_3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:background="@android:color/transparent"
        android:text="Area\nDocenti"
        android:textAllCaps="false"
        android:textColor="#030000"
        android:textSize="26dp"
        app:layout_constraintBottom_toTopOf="@+id/textView2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>