The only area where you can say the Allies were really 'outnumbered' by the Germans during
Fall Gelb was in terms of aircraft. The Allies had more tanks (about 50% more), more guns (about twice as many more), more divisions, etc. Also, the Maginot Line really wasn't that bad. It served it's purpose. Only thing is, the French were caught totally by surprise when the Germans went through the Ardennes.
Anyway, in the PC game Hearts of Iron, I've been able to hold off the Germans (when playing as France), in Belgium - and actually pressed deep into Germany by fall 1940. Of course, my 20/20 hindsight helps, in these matters.

Plus I like to start extending the Maginot Line all the way to Lille starting in 1936... that doesn't hurt, either. Then throw in some really good, concentrated tank divisions (alternate history doctrines), and you can start to do some serious damage to the Germans in Belgium. Also, take command of the Belgian divisions, and use them as fodder (as planned, historically).
But the Maginot Line stands there, with minimal manning... even still the Germans would be stupid to make a direct attack. Thus, you just have to position your army in the right place, and the historic 'attack through the Ardennes' maneuver gets stopped in it's tracks.
It ain't easy, though. You've gotta know what you're doing, and prepare your forces accordingly, starting back in Jan. '36.
And speaking of 1936, when the Hitler violated the Versailles Treaty, the French alone could have attacked and easily kicked Germany's @ss back in the mid-late 30's.