I don't think it's good to have this difference to normal C in such a basic functionality. Just imagine someone who used C already for some time and then switches to Lite-C because he wants to make games now. He writes some code and he will soon run into serious problems because sizeof works different. And the problem will be most likely some memory related crashes which sometimes may occur or may not (we all know this kind of problems I think).
And to say the truth, if your code has a problem, I think the last thing you would check is if sizeof is working correctly ^^

Thats why in my opinion, this is a serious problem...

Edit: Thanks Sid for searching the according paragraph in the standard!