Our plans include lifting so we like to lift some to gain some strength without too much bulk and for injury prevention. But for sure too much bulk will have some negative aspects. Muscle weighs quite a bit so having to carry big muscles in endurance events wil slow you down, specially if this muscles are not muscles that will help you power down the road. The other aspect is flexibility. Too much bulk can hinder your range of motion which is something needed to a great extent in some of the triathlon disciplines.
But most of us do triathlon for sport and enjoyment, and perhaps giving up something you enjoy is not worth it for the added speed on race day.
I hope that helps.