I am wondering how picking a big firm vs. small law office could affect the outcome. On one hand a big firm has more resources but I guess they are more interested in corporate clients with deep pocket, while the small office could give more attention to an individual inventor.
More relevant question is how to know patent attorney is efficient?
I will provide some points which might give you insights on things that matter.
First of all I would emphasize it does not matter Big or Small firm!! what really matters is whether my case is deeply understood by attorney. for example my invention is some sort of mechanical and thermodynamics base equipment and firm is specialized in biological invention (attorney if of Bioscience) which means that though they have extensive knowledge of IP rules laws, Inventor has to sit and make them aware about each aspect of invention. Even after that just a pop-up question by examiner may raise doubt in attorney mind. Just imagine that in a war soldier has a doubt on what to do? To my opinion what really matters is attorney is well versed with technology class you want to get IP. just search the patents they have represented.
I have not seen a single instance which says attorney has given less amount of time based on money or bill factor. Yes they might feel lagging in terms of technology knowledge but they try really harder to make inventor win.