How to name a mobile GPU:
1. Cripple the desktop part by half
2. Make it share the same model number as the desktop part, but add a "m" at the end
3. Profit
Your point is true with regards to power and specs of notebook GPU's compared to desktop ones, but in my opinion this is an unfair characterization of both graphics companies.
The 5770, which is by all standards a power sipping desktop GPU, eats 15-30W at idle and up to ~108W at load. Try putting that into into a notebook!
The 6770m, meanwhile, consumes about 30W at full load. That's pretty impressive, it's actually about twice the performance per watt of the 5770.
Sadly both nvidia and ATI have been misleading people for years this way, trying to make them think the xxxM is close to the performance of the desktop xxx that people see reviews for.
nvidia 560M? Not really a 560, it's slower than the desktop 550.
I would prefer ATI and Nvidia go to different names other than an "M" to differentiate mobile GPU's too. In the past, ATI used a 3 digit nomenclature for its mobile GPU's (and still does for some - the 4650m was rebranded the 560v), but both companies have recently been using equal names to their desktop chips. However, expecting a 6770m to perform the same or better as a 6770 or even a 5770 does reflect a bit of wishful thinking, that you could just take a 100-250W desktop chip and somehow magically put it into a notebook.