The sig rules are fucking daft anyway.
No they're not.
Not that it's worth discussing or changing, but the sig rule is inconsistent itself anyway.
Not really. You have to understand the history behind it.
Previously, the sig banner rule was 400x400 max. No flexibility. Several posters wanted sig banners that could maximize sig space in the rules, but still have them horizontal for aesthetic reasons. However, at the time the rules were rewritten, I wanted to ensure that those with lower resolution monitors would not have other people's sig banners break their forum layout. Thus, the rule became a total of 800 pixels, with the length no bigger than 500 wide.
But then argument came that some people may want tall sigs rather than long sigs (but I, and many others, didn't want to scroll through 700 pixels of someone's thin sig image), so the rule was adapted to 500 total in either direction, with a total of 800 max. This solves everyone's desires. If you want to max sig space wide or tall, you can have it. If you want to max sig space with a square, you can have it.
Obviously, as Lee mentioned, technology has changed and the number of 800x600 monitors is quite small. However, I believe one of the forum layouts still uses a fixed width layout, so there's no reason to change the rule to break the layout. Furthermore, if you changed the rule to accommodate large sig images, what's the point and where does it stop? Finally, limiting to 800 pixels width+height keeps sig sizes smaller, which has nothing to do with server, but rather image download size. Some people still run very slow Internet connections, so they shouldn't be deprived of forum usage because someone wants a massive Cesaro sig banner.
Presumably the rule is to do with server space
Not server space as much as Internet downloading of data.
as if it was aesthetic the 500 wouldn't be allowed in either direction.
You say that, but one of the primary complaints when 400x400 was the rule was that people wanted wider banners but still max the rule. It was a very real thing.
Also, I completely disagree that a perfectly square image is always more aesthetically pleasing on the Internet than a horizontal one. Most Internet websites agree with me as well, as do most photographs. In fact, I'm not exactly sure where you get the idea that a perfectly square image is aesthetically preferable to one which favors either height or width.
But this is ******ed - the space taken depends on the image type
It depends on several variables yes. But I think we would both agree that, no matter whether the image is a jpg, png or bmp file, a 500x300 image is going to a smaller file size than a 1500x1200 image of the same thing. Right?
but even ignoring that and assuming every pixel is the same, it's totally inconsistent 400 x 400 = 160,000 pixels. 300 x 500 = 150,000 pixels.
As I've explained before (whether it was you or someone else), the rules use the 800 number because it is far easier to understand. Sure, basic area math tells you what you posted, but can you honestly tell me saying "No sig bigger than 160,000 pixels, with one side maxing at 500 pixels in length" is easier to understand than what we have?
The sig banner rule makes perfect sense, once you understand from where it comes.