Production was based on need.
As the the war developed, the need for the Germans was to stop the Allied bombing offensive, thus planes that were faster and carried larger armament. This can be seen clearly in the development of the 109's over time, as well as (although to a lesser degree) the 190's through the Ta-152 and finally the 262 and 163 as intercepters.
The allies on the other hand, particularily the US in the European front, were concerned with range, speed and altitude as bomber escorts as can be seen in the development of the P-47's and P-51's.
The British were concerned with homeland defense which was epitimized by the Spitfire.
Similar need based development can bee seen through all the counties that participated in the war (Russia with exeptional low altitude planes to repel Germany's ground offensive for instance).
If the needs changed, then there would have been more development but in Germany's case in particular, the ME's were able to evolve to fit the need and the production facilities were already in place. There are considerable differences between the 109E-4 and the 109K-4 (or the 163 or 262). They aren't even close to the same plane. A true tribute to Willy Messerschmitt's design genious.
