[Klaus]

Here's a more or less fair comparison of fully equivalent setups:

MFT vs APS-C vs FF

 

http://j.mp/2qohuve

 

As you can see - almost no difference - as expected.


Note: The Pentax lens is actually a little "longer" (equiv. 375mm) which is why it is a little bigger.

 

Pentax 60-250mm f/4 (x1.5) = 90-375mm f/6 

Olympus 40-150mm f/2.8 (x2) = 90-300mm f/5.6

Canon 70-300mm f/4-5.6 = 70-300mm f/4-5.6

 

Same quality league.