I'd hardly call it stupid to be honest. You sound like you're fairly well informed already and I'm not sure I can really say anything particularly insightful.

Bilinear happens to be the most commonly used resize filter (outside of the anime fansubbing community). I've never used debilinear, but if it does what it says, it would presumably restore the "original" image more accurately (note, "restore" isn't necessarily the same as "better perceived quality") than other resampling methods... assuming that the original was upscaled using bilinear (which probably isn't a bad guess).
Spline36 would be better for generic resizing.

The two methods would probably look a bit different but it would be difficult to say which is exactly better. I checked the screenshots posted at Nyaa: to me, the debilinear'd version looks sharper but has a bit more noise. You could probably get a similar effect by applying an unsharp mask, or some other sharpen filter, over the spline36 version, if you prefer the sharper lines.
Okay I lie, the text in the debilinear version does look better (but it looks about the same when I add an unsharp mask to the spline36).

Personally I'd stick with spline36 or similar and apply a sharpen filter, or use a sharper scaler (eg spline64), mainly because it seems safer. Then again, I've never used debilinear, so take this with a grain of salt.
Pointless conclusion: At the end of the day, whatever appeals to you.

I can't explain the size difference though. If the mediainfo is correct, the encoding settings seem to be the same. I can see the noiser 2nd version being larger, but I wouldn't expect the difference to be that big. Of course, this is just a wild guess/hunch without actually seeing the video itself.

The fansubbing community is probably one of the most thorough and critical bunch of people, in regards to releases, out there. It's perhaps a bit surprising if you think about the fact that anime is probably more difficult to release than, say, US TV.
This thoroughness is a good thing, as it means we get good quality stuff. Some issues come out of it though, such as going over the top or nitpicking things, but I think it's ultimately not really an issue to be able to get what we can get.