I didn't think it was to do with metabolism, I was under the impression that large insects and arachnids simply can't breathe the insufficient oxygen content that we have here on earth at present. Oxygen was supposed to be in greater abundance on earth millions of years ago, and that's when we had dragonflies with 6 foot wingspans. Something to do with the exoskeletons.
However... we have Giant Jumping Spiders and Giant Brown Recluse on the surface so I think it's safe to say that whatever happens Oxygen isn't an issue in Dwarf Fortress.
This would be post oxygen catastrophe, in the midst of the carboniferous era.
The rise of single celled photosynthetic organisms culminated in a massive explosion of evolutionary potentials, as a vast stream of metabolic energy was suddenly made available in the primordial high co2, high greenhouse gas earth atmosphere. the consequence of which was a radical pendulum swing in atmospheric concentrations from heavy co2 levels, to heavy o2 levels, which had a poisonous effect on many early lifeforms. Hence the name, "oxygen catastrophe". The following era of geologic history is the carboniferous era, in which early animal forms capitalized on the massively vegationally dominated landscape. Atmospheric oxygen levels were so high that fermentation and biotic decomposition ran serious risks of initiating spontaneous combustion, lightning strikes almost always caused wildfires of insane proportions, and insects were the size of turkeys. (Well, a 'slight' exageration there...)
Nothing nearly the size of a GCS though. The GCS would either need a radically different biochemistry (sulphur cycle perhaps.... close proximity to volcanic vents would make it a viable adaptation), or a well developed respiratory system. (Doesn't need lungs per se, but an an analogous tissue on flapped "gills" like those found on shrimp and lobster would suffice if kept sufficiently moistened.)
Without such a radical adaptation, the beasties would need to be retardedly sedentary, with a rediculously efficient and slow metabolic rate for passive osmotic oxygenation (what is currently present in insects and many arachnids. Some arachnids have "book lungs", but are not efficient enough for a species as large and active as a gcs.)