Well as far as research efficiency and such, Hooman posted this info:
http://forum.outpostuniverse.net/index.php?showtopic=3315(which was probably the original reason it works that way, for gameplay purposes).
One other possible way to explain it could be wages for the scientists. The scientists are probably getting extra ration credits as payment for their work (the novella and the "Tales from New Terra" stories in the help file mention a credit system) and thus if they can do with fewer scientists, they save credits (which indirectly translate into consumable resources like food and such, things they would have very little of)
Or perhaps you could consider the whole idea of diminishing productivity if an excess number of people work on something. For example, if 1 man tries to build a house, it takes him a long time. If he has 4 helpers the house is built much faster. If he has 500 helpers virtually nothing gets done since there are constantly people bumping into each other and getting in each other's way. The same could apply to simpler research in OP2: if they have too many scientists trying to work on the same project, there simply aren't enough jobs for everyone to do on a project, for example Metals Reclamation. You would have extra scientists sitting idle or not putting out maximum productivity. But then take a research like Space Program, there are hundreds of subtasks to complete so just about everyone can get in on on some part of the (comparatively much more massive) project.
The bottom line is, we will probably just have to accept that things work this way for gameplay balancing reasons. If you wanted to think about the research model it is completely unrealistic (since the scientists aren't specialized into different fields by the University training, only training 10 scientists at a time, rigid techtree that is the same everytime the game is played, etc), but it is this way to make the game playable / fun.