I can't remember the name, but I know somebody did a fan-fic that addressed what happened to the various adults on the way to the Great Valley. I only remember a few elements - Nod's father got killed shortly before arrival, Topps gradually lost most of his family to incidents along the way, etc - but I believe at one point a pair of Spiketails were mentioned; the female was grieving, and the male explained that he'd forced her to run for it and abandon their egg when the Earthquake happened. I don't remember what happened to them; I should seek out the fic again!
Although at least one storybook suggested that Spike's parents made it to the Valley and he later found them by following their large footprints, every other material suggests they died or went somewhere other than the Valley. Since it's unlikely that a parent would abandon a child like that*, it could be that they were killed by a Sharptooth, but they intentionally led the Sharptooth away from their nest so their child would have a fighting chance.
*Remember, Littlefoot and Cera were on the wrong side because they were fleeing from Sharptooth. Ducky's family likely stampeded and didn't realise they'd lost a family member until they stopped. Petrie...of course he was left behind because he couldn't fly away, which suggests they left as a result of the Earthquake. Being able to fly, they would have been able to return for him after things settled down, which suggests that Petrie fell from their nest and became lost amongst the rocks - when his family searched for him, they couldn't find him and were forced to move on.
As for the size change, yeah, I think that's an error. Although, Ducky did remark that Spike was a late-bloomer (perhaps his parents thought his egg was a dud and left?). Perhaps he's been lazing in his egg so long he's turned out way bigger than a normal young Spiketail. Also, I suppose a Spiketail's metabolism allows them to grow quickly by eating more? Could explain why Spike's always eating - it's an instinct to grow up and get bigger.