The Celtics can't beat the Cavs(even though they did the last time they faced) not because of size or jumping ability but because of talent. When these two teams take the floor against each other, the Cavs have the three best players on the floor and one of the stronger rotations in the league. They are simply a better team than this current Celtics team.
Now in a year or two, I expect the Celtics to be the better team, but for right now the Celtics lost tonight not because of height or length or size but because they just aren't as good as the Cavs.