Remember, to a hammer warrior, everything is a nail.
My wrathful hammer runepriest would most definitely agree!
As mentioned above, I think its fine to have certain monsters be immune or resistant to certain schticks. Heck, Tiamat could be granted a save vs. prone (a save she'll make 80% of the time given her natural +5 bonus btw). But in general, I do agree that you shouldn't be routinely screwing over a PC's theme just because.
In a bit of irony, while I realize the idea of a human, halfling, etc. knocking over such a large creature seems unlikely, look at how this will affect the PCs over the course of the campaign. During heroic tier, its unlikely to have too much of an effect. Most of the monsters they fight will be medium or small (with a few large critters thrown in). As they become more powerful though, they'll be fighting more and more large creatures and as they head toward epic even huge creatures become a lot more common. The irony becomes, as the heroes become more and more godlike, they become less and less able to do their schtick if we were to just rule "You can't do that because the monster is too big, etc." In fact, this type of ruling could actually render some higher level powers more or less useless.
In fact, I think part of the reason that WotC assigned power sources to all classes is that there's this idea that even for the dumb fighter, its more than just practice. His training and natural exceptional ability has allowed him to tap into this source of power that most people can never dream of being able to touch. Its this power that adds that extra bit of oomph to his attacks. If you really need a logical explanation, there it is. The real fact of the matter is that the rules in any RPG can only ever cover so much real estate. We are playing a game where wizards, elves, dragons and vampires don't cause anyone to bat an eye. If we're willing to accept that, why can't we accept that a fighter might be able to put a dragon off balance with a single blow? If we wanted the rules to cover every possible scenario in a completely realistic manner, the books would be thousands of pages. Revflavor it if you need to, but don't make rulings that essentially say "Prone is essentially ignored by the bad guys once you reach a certain level -- though they can still knock the PCs prone."
That being said, its still perfectly valid to mess with their schtick every now and then.