It was likely a different treestar. The first treestar was as wide as he was I believe, and then it disappeared after the whole sharptooth+earthquake incident. It would be strange if that one treestar was the same one his mom gave him. But if it is the same, only thing I can think of is he got a growth spurt.
True, Littlefoot could have grabbed the treestar, but he was frozen in fear. He waited too long to react because of that fear, and by the time he could get himself to do anything, it was too late.
As for Littlefoot being mean to Petrie, you have to understand that it reminded him of his mother. Yes Petrie didn't know, but you can't blame Littlefoot for being upset. Petrie wasn't angry at Littlefoot; he felt guilty about it, and even tried to protect it afterwards. Also, Littlefoot didn't know Petrie that well, so he likely didn't know how to properly respond to him as he probably didn't encounter someone like Petrie before.
Littlefoot hadn't had much social experience outside of his family until recently, especially with youngsters around his age. That scene is from a very early part of his childhood were he was starting to learn about friendship and how to behave around others. So if you believe Littlefoot was unfair to yell at Petrie like that, you could just say that he didn't know any better at the time.