Think about how many objects end up being created in a typical blockland server. I think the brick cap is 128,000? In Source, try spawning 1,000 crates. Like, take the stacker STOOL in Gmod and spawn a crate, stack ten horizontally, ten up from each of those and then ten out from each of THOSE and see if your computer doesn't crash.
source is terrible but physics objects don't necessarily need to be affected by physics; frozen props in GMOD are an example of this. they do not lag at all and the only thing limiting how many you can place is your FPS and the source entity limit.
Also, Source and I imagine Crytek are not built for expansive spaces map wise.
source is total garbage at open areas and it's tiny, something like a half mile^3 maximum
crytek is a company. cryengine is the engine.
crysis has some huge levels, what are you talking about.