The decision to allow users to upload any size images was one made to promote ease-of-use. Considering nearly half of the visits to GTP are on mobile browsers these days, an easy-to-use system that encourages users to stay on the site made sense, and still does for large chunks of the site, where this is less of an issue.
You won't hear me disagree about it being a problem within the Photomode section, though; this is a specialized area. I was relatively slow on the uptake for Flickr, but I'd gladly sing its praises to those not in the know these days, as it's a high-quality, free alternative - and provides another avenue for community exposure, really. I also can't stand seeing images dumped straight from the PS3 to a thread - they muck up the page layout and the click-to-full-size feature does the same thing. But the cream rises to the top: the folks that produce thoughtful, user-friendly photo experiences tend to get greater results, and those people too lazy to consider others' experience, less so. It's the same mindset that steers the competitions: sure, people are fully allowed to post tiny thumbnails as their final entry, or not have their 500px image linked to a full-size image... but if they can't be bothered to produce an entry that will give them their best chance, I'm sure not going to do the work for them. Any non-clickable images are put in the poll that exact same way.
It's an unfortunate truth that as access speeds have picked up over the past few years, the majority of people have cared less and less about file sizes. I'll flat out avoid the non-gallery threads when I'm not on a desktop computer because of that, and yes, a page with well over 100mb of files is not good for anybody's connection, regardless of the formatting havoc it wreaks. But as people who care about things like pixel counts, page loads, and formatting, we're in the minority. We need to educate.
But the tone has to be positive, and it needs to be a community-wide approach: when a member hops into a comp thread with an improperly-formatted post, often times before either Nato or I can get to it, another member has steered them towards proper posting etiquette, in a way that is far nicer than the typical "read the FAQ noob" response that can rear its head in other areas of the web. The OP certainly doesn't come off as particularly friendly.