I don't think any of these are under 500 mb. Try Epic Conquest and Epic Conquest 2. Also, Heroes and Villages and TES Blades. And Final Fantasy Pocket Edition.
How about old game, Stone of Life (it has sequel) and you can play it offline. You can get a gem each hour at the title screen. But as old game it need "Google Play Game" which modern game dont use it anymore
Honestly, just modded my ps vita recently, soo many rpg's on the system. Best thing about it is I have access ps1, psp, psvita, n64 games [emulation], all in the palm of my hand and it works great! Would highly reccomend the Tales or Dark Souls series if you're on PC though.