We don't have control over this as we don't make the rips.
It used to be that filesizes were set to a hard limit (CDx1 size and CDx2 size). The old .avi rips followed this standard (1.4GB, aka 2 CDs, for the 720p rips). This is obviously an outdated rule because who burns things to optical media anymore?
I believe the current release rules (for the better .mp4) are based around a specific resolution and bitrate cap. The bitrate is dynamic though, lowering itself when it can in cases where a high bitrate is not needed. Some shows/episodes need to use a bitrate closer to the cap though due to what's actually in the video and that makes for bigger filesizes.
Or at least that's my understanding anyway.