Well, Tidus' sphere is the same size as those in his game, so not all of them changed size. I think WoL's and Cecil's may have also been the same size.
And, really, Cloud's crystal is the right size to be any materia but the White or Black Materias. Well, not the Goddess Materia either, of course, but all the rest of them.
WoL's is the appropriate size for their handheld crystals.
Firi's is IIRC, the right size- we've never seen a full sized Crystal.
Onion's is shrunk.
Cecil's is shrunk, but by how much depends on the version in particular.
Bartz's is of appropriate size.
Terra's is also of appropriate size for the Magicite we've seen.
Squall's... Yeah. No clue what it is, but it's certainly not an expanded version of anything we know.
Zidane's is shrunk considerably.
Tidus is of appropriate size.
None of the other crystals are 'expanded' in any way shape or form, so why the fuck would Cloud's be an expanded version of something when none of the others are?
Which is to say nothing about how it fails to look like the white materia in other regards.