The only size-related optimization that I can think of is that V8 doesn't inline functions (but does optimize them) when they are more than X characters long, c

The only size-related optimization that I can think of is that V8 doesn't inline functions (but does optimize them) when they are more than X characters long, comments included. I'm not sure about SpiderMonkey, but they may very well use this kind of optimization too. However, I would guess that Uglifying files should not increase execution speed for any noticeable amount.