Well, the jumps are as small as they can be. The only one of them that might be a little "heavier" is the big metal one, but you can just not install that one.
Arena fences are also as small as they can be... The arches are only made up of a couple pieces... Unless you lose the shape completely, it wouldn't be possible to make them smaller (aka not without making a completely different mesh).
The only thing that could be made smaller are the bales.. I included them more as a 'request' than a plan, so as of right this minute, I'm sorry, I'm not planning on working on them. The individual bales however are not that big. I would just not install the stack if your game is having a hard time because those are indeed VERY big (despite me deleting most of the non visible joints).
Other than that, you might have to pick and choose what can be in your game at a specific time (this is what I plan on doing myself, so that my game runs faster : ) )