While that exact realization did make me stop being freaked out by giant spiders in movies, technically "they can't breathe"
in earth's current atmosphere. Presumably, if it was worth the expense involved, you could grow giant insects in closed biomes with higher oxygen levels and (I assume) higher atmospheric pressure.
Also: Jurassic Park is BS for the same reason. The T Rex would not function in earth's current atmosphere -- at least not without tampering with their physiology and/or some kind of extra support system.
