With respect, Defender, I think you're wrong. We could imagine a mech game developed in which it was said that, since it's science fiction, all houses in the future are built to look exactly like normal modern houses - but are in fact ten times as large. Similarly for cars, roads, lamp posts etc. This would result in the mechs feeling to be about 1.2 m high in comparison. Say about the size of a hobbit. Of course, the devs could respond to people who complain it just reflects a lack of imagination on their part. But in truth they would have shot themselves in the foot by undermining the sense of scale. I don't think it's any different for trees. Is there a technical reason why the trees only come in size XXL? I can imagine that maybe mech/foliage collisions cause problems and the only solution is to make the trees large enough that the canopy is above mech height.
Anyway, it's up to you lot and I'm sure it will be fun to play whatever decisions are taken regarding relative scale.
Cheers,
RD.