Hey HalfLoaf,
In the case of GamersGate, they may be delivering an old version of the installer, or that installer may be right but their stated filesize is simply out of date. Either way it's no problem to buy from them, they send you a license key and you can just plug that into the version of the game you already have installed -- no need to re-download, the demo and the full version are the same thing, with just a license key activating the full version. There is also a Check For Updates button that allows you to get the latest version if the GamersGate (or any other) version is out of date. But I think GamersGate is presently on 2.0, I am pretty sure that is just an outdated file size.
For Impulse, the larger file size is based on it downloading all of the prerequisites -- normally that only happens if you actually need the prerequisites. The .NET Framework and DirectX and SlimDX are what make up the difference in file size with them.
I've never heard of Paypal having a surcharge for purchasing things from Steam. It might have to do with non-US orders, I'm not sure. But, Steam has ways of paying direct with credit cards, or other options, which I think would let you work around that. But I'm not an expert, that's more a question for Steam support -- they do an awful lot of international business, though, so I can't imagine there are extra fees with every transaction, as that would really crimp them.
For GamersGate, or our site, or I believe Impulse, it will quote you in GBP if you're in the UK -- and yep, that's all you'd pay unless the e-commerce site calculates you owe VAT or something. Easiest thing to do is just put it in your shopping cart and see what it quotes you. If GamersGate calculates that you owe VAT on the transaction, then most likely any other ecommerce partner selling games also would. That varies by country and possibly even by state/province/region in some cases, though.
Hope that helps!