I'm pretty sure it's something to do with the chemical composition of the plastic. The Twinkle Eyes seem to be prone to the rock hard body, while foreign ponies tend to be squishier. I haven't noticed it as much with the G3s, but, say... *squeezes Trickles* hard *squeezes North Star* soft. Different batches of plastic, I suppose...
Getting them hot also makes them squishy, as my friend found out after she'd left her ponies in the car too long with her luggage after coming home from college. They're pretty much back to normal now. :tonguew: