I'm sure designers and site-builders do think about file-sizes. If they don't they should be Search engine's will penalise slow sites in their rankings so having a 700kb image is just stupid. If you downloaded that on your phone it would practically be indescent (my poor inadequate tarriff).
So what is a good size? What can you do make things smaller (and I don't mean stand in the cold). I thought it was probably just smaller image sizes, good compression, simple design. A quick Google search tells me that, as usual, things have moved on since 1995.
Check out Yahoo's tips on speeding up your site.
It talks about everything from managing cache, using content delivery networks, how you code, etc. Optomising images is just a tiny tiny bit of it.
Anyway check it out and consider if you need to start putting your website on a diet.