Bitcoin chooses the "best chain" based upon the one that has the most cumulative proof of work behind it. Are you proposing that the cumulative proof of work be ignored if two blocks are within a certain threshold of each others' work and if so, the number of transactions in the block / the size of the block should be used as a "tie breaker?"
I think this idea needs more fleshing out of exactly how it would work, with careful consideration that adding complexity to the best chain logic could introduce exploitable flaws.