I think teams that have larger attendance numbers due to having a bigger park, often paid for by the community would disagree.
The parks smaller than ours, MIA, CLE, ATH & TBR all are very low in attendance numbers, but a bigger park would not help them, unless a location change might be part of the upgrade.
PIT, KCR & MIN are just above our capacity and would also not likely benefit from having parks 5-10K larger.
The Dodgers have almost 8K more capacity than the #2 team, and it helps them make more revenue than anyone else- that and their fat TV contract. AZ, SEA, COL & NYY are next and all over 46K. All but the Yanks have lower attendance than the Sox, so you make a good point.
The top avg Attn teams and their rankings on park capacity:
1 LAD 1
2 NYY 5
3 SDP 21
4 PHI 10
5 NYM 11
6 CHC 13
7 ATL 16
8 SFG 15
9 BOS 26
10 HOU 16
The thing is, teams #9-22 in capacity are all within about 3K seat from each other. (39K to 42K) They all can get 2-5K more per game and often do. Many still end up below the Sox, because they have many games under Fenway's capacity.