...and here's where I reveal that they're all gremlin sized.
The world of Cibarium has an atmosphere much higher in oxygen than our own. That's important for sustaining large terrestrial arthropods because they don't respirate the way humans do; insects and arachnids have openings in their bodies through which air travels passively to oxygenate their blood via a network of tubes. It's an ingenious adaptation, but as a result they have a limited ability to separate oxygen from the atmosphere, and thus a limited body size.
The hyper-oxygenation helps but there's still a clear upper limit to what your exoskeleton can support, especially when you have an upright posture. Arthropod muscular systems are not terribly advanced, so there's only so much they can do to stop the joints from buckling under the weight.
I also decided to make them like this because a three dimensional structure big enough to hold an entire city gets a lot harder to explain when its citizens are all of average human height.
On a semi-related note: because of the high oxygen content, any flame larger than a candle is extremely dangerous and has been explicitly outlawed by the country of Labrum (the country in which Cibarium resides).
None of this should really be taken as scientifically accurate, I just like being a nerd lol
