Personally, it bugs me too
1
that aside, think of it not as a "1st level at-will" pushing the dragon, but as a "15th level character" pushing the dragon.
Since the PC has to hit the target defence in order to make the push happen (I'm guessing), a huge dragon is probably a higher level target and has much higher defences than, say, a kobold. The 1st level character is very unlikely to hit that defence and so can't push the dragon. The 20th level character is very likely to hit the defence and thus move the dragon via leverage, pressure points, chutzpah or whatever.
The level of the power typically determines the scope of the effect. The level of the PC determines how tough a target you can affect with it.
1. In particular, it bugs me that dwarfs get pushed one space less but nothing else does. The next time I run a campaign there is going to be a house rule in there that states every size category above Medium causes a creature to be pushed, pulled or slid 1 square less by any forced movement that results from an attack against AC, Fort or Ref.
Forced movement that results from attacks against Will are fine, as they are causing the target to 'choose' to move.