I created a fleet of 5 cruisers armed with WMGs and pitted it against several enemy cruisers with no weapons (to make sure I didn't lose my fleet with experience
). It took 41 ships killed to get fleet experience up to 1%. Every ship had at least 4% ship experience.
I took a fleet of one frigate armed with a WMG. It took 40 of the same cruisers to get to fleet experience 1% (ship experience 40%).
I took a fleet of 19 of the above frigates armed with WMG. It took 47 of the same cruisers to get to fleet experience 1% (ship experience ranged from 0% to 4%).
Note that it took several battles to get the above results.
A battle between the 19 fleet (reconstituted with 0% fleet experience) and a group of 80 cruisers generated 2% fleet experience (combat turns extended to 999).
A battle between the 1 fleet (with 0% fleet experience) and 80 cruisers generated an integer overflow.
In other words, the answer is that fleet experience does not rely on tonnage, damage, fleet count, ship experience, or any other rational explanation. Rather, it is a random function triggered by unexplained cosmic phenomena.