The Firenze, High Resolutions and ~ WIDESCREEN SPECIAL EDITION ~ themes were all created specifically for resolutions like yours and use 75% of the screen width instead of 100% (80% for Firenze I believe). I don't really understand why you find them look cramped, because making it wider would make the difference with the standard 100%-width themes rather marginal.

User experience and scientific research indicated that at horizontal resolutions of 1280 and such a 100% wide page does not make reading the page a very pleasant experience. Therefore the themes for higher resolutions generally reduce page width so not to make lines too long. If you disagree with this principle, feel free to use the normal themes

I'd suggest you try them for a while, it takes some time to get used to.