According to this:
It is important to note that this is very different from most SCM systems that you may be familiar with. Subversion, CVS, Perforce, Mercurial and the like all use Delta Storage systems - they store the differences between one commit and the next. Git does not do this - it stores a snapshot of what all the files in your project look like in this tree structure each time you commit. This is a very important concept to understand when using Git.
Yet when I run git show $SHA1ofCommitObject...
commit 4405aa474fff8247607d0bf599e054173da84113
Author: Joe Smoe <joe.smoe@example.com>
Date: Tue May 1 08:48:21 2012 -0500
First commit
diff --git a/index.html b/index.html
new file mode 100644
index 0000000..de8b69b
--- /dev/null
+++ b/index.html
@@ -0,0 +1 @@
+<h1>Hello World!</h1>
diff --git a/interests/chess.html b/interests/chess.html
new file mode 100644
index 0000000..e5be7dd
--- /dev/null
+++ b/interests/chess.html
@@ -0,0 +1 @@
+Did you see on Slashdot that King's Gambit accepted is solved! <a href="http://game
... it outputs the diff of the commit with the previous commits. I know that git doesn't store diffs in blob objects, but does it store diffs in commit objects? Or is git show dynamically calculating the diff?
No, commit objects in git don't contain diffs - instead, each commit object contains a hash of the tree, which recursively and completely defines the content of the source tree at that commit. There's a nice explanation in the git community book of what goes into blob objects, tree objects and commit objects .
All the diffs that are shown to you by git's tools are calculated on demand from the complete content of files.