How big is too big

[Sodomizeher]

No issue with file size... I have an 8TB NAS for all my porn

8TB of dilatated gory orifices. yay Even my collection of torture porn is not that twisted

It's not full, not yet anyway

[Mr. Peter]

Seeking performance should be more or less (wm9 is tad worse) the same (in respect of the resolution). If not, the content is not
properly encoded (mostly true for files encoded by substandard not a pro sw encoders or using inappropriate settings).

MPEG-4 (h264/aac) has a lot advantages over Windows Media.

It depends on the angle of view.

-codec efficiency? No. (See whitepapers or tests performed by DVD Forum)
-visual quality? In doubt. (h264 advanced profile gives comparable IQ for high res content but at impact on coding power).
-versatility? Not sure (compatibility issues across equipment with coding h264 features, VC-1 standard is more strict)
-better / wider support by adopters? Lately definitely yes in both pro and home area. Dont ask me why

Anyway both were designed for different purposes. WM9 (VC-1) was designed to maintain IQ at all costs since
h264 was designed to be as much as efficient as possible compromising quality in complex situations. That's why it's often used in ultra low latency hi def content streaming systems. On the other hand where IQ is of a concern, VC-1, JPEG200 or even MPEG-2 at insane bitrates (over 150Mbit) can be used.

A note to the file size. While MPEG-4 bluray's bitrate is 30Mbit on average, for VC-1 you can go as low as 10 or 15Mbit to give the comparable IQ.
That's about 3GB per 30mins of movie.
BUT #1 That applies for mainstream movies shot on professional (digital or film) cameras using lenses that cost as much as a new family car.
BUT #2 porn (especially online content) is shot now in 99.9% on low cost digital cameras - a semi pro category at its best (21sextury for example).

So there is really no need for several gigabyte files for one porn scene in 1080p format. Anyway for internet I'd choose 720p at a slightly higher bitrate which will result in a smaller file and sharper and cleaner image. (Most users won't recognize 720p over 1080p, but they recognize blocking, colour changes, banding, softness in 1080p encoded in low bitrate to keep reasonable file size..)

If you would be shooting porn with Red One, Scarlet, Cine Alta or scanning 65mm film in 8k...that's a different story