My Head-canon for spacecraft Transformers. Space-travelling transformers like Omega Supreme or Astrotrain have the ability to switch between human-sized and transformer-sized, which explains how Astrotrain could house an entire army in The Transformers: The Movie. Earthbound transformers with car or plane alt-modes are only limited to their normal size, but spacecraft alt-modes are equipped for both human-sized peoples and cybertronians.

Yes, there were instances where transformers with car and plane-modes could fit other transformers (Thrust, Huffer, Cyclonus, etc.), and handheld alt-modes could fit in human hands (Megatron, Soundwave, etc.), but I would prefer if only spacecraft alt-modes could switch between human-sized to transformer-sized.