There are three factors that correlate to the reward you earn: Experience Level, Force Points Earned, and Task Force maximum Capacity. Here are the relations.
1.) The greater your experience level, the greater the reward if Task Force Size and Force Points made remain constant.
2.) The more Force Points the Task Force earns, the greater the reward if Experience Level and Task Force Size remains constant.
3.) The smaller the Task Force Capacity, the greater the reward if Experience Level and Force Points earned remain constant.
The amount of Intel you gather does not correlate to the reward. Still, I do not know the effect ratio between Task Force size limits when other factors remain constant, so I am unable to answer your second question. Sorry. :(
Something I have been trying to figure out. My ten man task force beats let us say powder keg for 415 force points for 41.5 each member. If we expand to 25 for a similar ratio would we have to beat operation giving 1100 force points to get an equal reward. If it is based on force points divided by task force size then a five man tf all hq 21 being equal that can beat upperlip then their rewards would far exceed a 50 man tf that can beat massive attack because their force points per member would be much higher on the 5 man vs the 50 man.
Maybe make a 5 man tf and beat one of the small operations and write all stats down for the members rewards received then expand to 10 and run an operation with twice the force points and see if there is a change or if everything remains the same. I am on a good ten man that beats mambo 4 out of 5 times. I can be sure if we expand to 25 it would be very unlikely we could beat choke point to receive equilibrium force points to member ratios.