So I have two workbooks and they both have the same number of worksheets and almost same amount of cells with data, but the second weights 12 times more.. What might be the reason behind this?
Most likely cause is excess used range. Check the last used cell on each worksheet using Control End (but note this only shows the last visible used cell)
Then delete all rows and columns in the excess used range (but note that this may impact any formulas referencing beyond the last data cell)