I only cook them in the sauce for about 10 or 15 minutes. Mine are always the size of the inside of my palm, so that's not huge. And if they're a little too mushy inside, maybe you could use a little more breadcrumbs to dry them out more. If you're scared of too much spice you could use unflavored breadcrumbs or just chop a couple slices of stale bread. They should hold together pretty tightly in your hand.
yea, i cooked mine about 15 mins in the sauce. i think the size is my issue, i think mine were bigger than that.