It depends how the records are made. (production, mastering, material etc)

7"s are more prone to Inner Groove Distortion.
12"s The grooves are farther apart & it can be mastered at a higher volume.

Music often get's split on a 45' as the track is to long to fit on a single side (part 1 & part 2)

They are a lot nicer to carry about. I love the little labels & art work. A sh*t tonne of music is only released on a 45, same for 12"s & cassettes!

It's all about the music.
I get annoyed at the pointless reissues and ludicrous prices. 7" records are cheaper to make!!