The idea of carnivorous tadpoles is wonderfully bizarre (although in my imagination they also grow to be a metre long, and leap out of ponds to bite peoples' arms off...)

Another possibility you could factor in might be that Suvolte demons live for a thousand years, and while they do produce dozens of offspring every time they mate, they only mate once every hundred years... ?

And perhaps the Suvolte in the episode ran off because guarding its eggs took priority over rampaging through downtown Sunnydale...but somehow, 'The Doctor' had stolen the eggs (in his TARDIS, presumably) and so the Suvolte was desparately running around looking for them again?