Fansub encoders usually use a constant rate factor (constant quality) and not a constant bitrate. This causes file sizes to vary for the reasons explained by Leggy.
Streaming services (CrunchyRoll, Hulu, Daisuki, etc. — files ripped by HorribleSubs) on the other hand use a constant bitrate, leading to very uniform file sizes.
You could always use Hiryuu's version until FFF's is released. I did have some weird subtitle issues in it though (the subs were inconsistent until after the opening).
Dear, Nihonjaki90. Would you reupload both of these please? I'd like to confirm the differences between them myself. Thanks. 交姦条件 getchu ver. 交姦条件 dlsite ver.