Parsing Blend File

I have coded some parser in c++
I compute the length of many struct without problems but i got problem with:
DrawDataList i compute length 16 the block length is 72
elsewhere
DrawDataList i compute length 16 the block length is 12 !
DrawDataList i compute length 16 the block length is 4 !!!
PreviewImage i compute length 48 the block length is 64
bNodeTree i compute length 492 the block length is 520

Some issues with padding?
Is the default scene cube on 3.4.1
the file reopen with no issues on my blender