Kinematics knows what he's saying... but that said, I'm not considering filesize too much for these encodes, mostly because I don't expect the average AMVer to know farther than zarxgui's defaults and such, so I personally am going to consider filesize as a negative point only once we hit really ridiculous amounts... but that's because I'm trying to be nice.
In the end, there are things more important than filesize and quality to consider in an amv, so I won't start detracting for those unless it's painfully obvious that the quality is subpar/the filesize really is too big. I wouldn't want quality to be the
key factor to put one video higher than the other ─ but it is still something I consider and that can put a video at disadvantage. I just don't want to be too harsh when there are more important aspects to an amv to consider, that's all.
That said, we also have to consider another thing about filesize: AMV visuals tend to be a lot busier than what an anime normally has. Sped up coloured footage and constant action and a lot of visual changes for the whole duration aren't exactly uncommon, and that's going to suck extra bitrate easily. So even at crf 17-18 it's possible to get past 4000kbps at 720p24. Hell, without tweaking x264's settings, a crf20 is prolly gonna push the filesize past 10mbps if you put some heavy static/noise effects for a while.
And while it's possible to optimize the settings, I don't think this is knowledge we should expect the average amver to have, since the focus for an amver is the editing, not the encoding.