Alliance size impacts several elements in the game chief amongst them the alliance ranking in the leaderboard.
Larger alliance => more players => more trophies => higher rank
Higher leaderboard ranking is perceived as more attractive and therefore makes it easier to attract “high demand” players.
This slant towards the larger alliance is not going to go away. It provides an incentive for alliances to grow larger.
As you near the TOP of the leaderboard and the alliance levels cost upwards of 150m. It become a central issue you live with everyday.
I can’t say I like it but I accept it and and I am doing everything I can to grow my alliance as quickly as possible.
With differences in alliance size, a huge range of player levels, multiple time zone, differences in ethics, free vs gems use, activity levels, wide ranging time commitments, balancing this game must not be easy. Given that the alliance level differential is not going to go away where it hurts the most (rankings, boosters), I’m ok with living with it here too.
The worst thing that can happen is to introduce new slants that provided conflicting goals, ex: high level kings being at a disadvantage, people emptying alliances to avoid being attacked, people fighting for other alliances during the final minutes of a round.
The case you describe (a 13 member gap) is pretty extreme, that must have been very painful. Given that you wrote this post, I will assume that you lost fiefdoms (like I did) in the current round. You will probably be matched with alliance with “weaker” alliances in the next round. From a starting point where initial fiefdom were distributed based on trophies, I suspect alliances will gravitate to their relative “strength” in terms of alliance size, activity level, player strength, king level, etc.
Good luck in your next rounds!