Gmod crashes when I add something massive

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Gmod crashes when I add a massive object or some dupes, I was having a problem at first to even load single player but I fixed that, But when I get on it I add like a Combine Mech machine, get in it, and everything is doing well right? But the second I shoot a missile it completely bugs up and crashes, also for dupe props, added a HUGE prop not realizing how big it was, and it crashed the game regardless undoing it.