It really depends on how you look at things, both nations have by far the largest number of options prospectively available to them and yet don’t even receive their fair share of available designs that get added, let alone if we were looking at the percent of of overall additions, even if you don’t count duplicated vehicles in other trees outside of their design nation, let alone specific variants and modifications.
Part of the issue is that the limit of ~20 or so vehicles that get added each patch are now spread across 10 nations each with a number of Trees that they could fit into, and of those not all of them will be suitable to any given player so the number of relevant additions per patch on average has reduced over time as nations and features have been added. Take for example the B-57B to F-105D gap it lasted [2495 days; 6 years, 10 months] it took a while for the RP cost of modifications to be reduced, I doubt that we would see the B-57G Tropic Moon III (first aircraft to be able to self designate for SALH ordnance) which would be a nice addition though anachronistic at this point considering what it would be up against.
Or that they deliberately hold back additions, and features to artificially induce attention ( F-86F, F-86K / F-86D, A-4M /A-4N, AV-8B(NA) / Harrier GR.7, AV-8B+, (US) F-4E & AN/AVQ-23 & -26) or try and double dip (A-6E TRAM vs F-4S release patches) by creating obvious flaws in lineups and mechanics (AGM-65 and the implementation of Contrast / Correlation seekers)
Putting aside instances where implementation is straight up broken (e.g. AN/AXX-1 TCS on the F-14B, AGM-65, M1A1-KVT, etc.) or abstracted (Radars & SARH mechanics have gone though how many revisions at this point?) it’s pretty obvious at this point. That some trees are causing issues (Japan, Sweden, Israel) and Gaijin’s seemingly doggard outlook on keeping the stream vehicles flowing, to keeping wales interested with a Meta addition and shifting lineups around.
Personally if it were up to me we’d have stopped adding nations with either France or Italy, increased the number of subtrees and only now be reaching the late 60’s early 70’s designs across the board(Air, Land, Sea, Rotor, etc.) at around the same BR to ease people into the shifting meta as reliable missiles are introduced.
At this point think the best way to look at it is that it’s probably for the best that they are rushing for AMRAAMs, and managed to avoid adding the F-15E(X) /Su-30 or F-22 so we still have at least a patch to go before things become much more frustrating(Arcade is going to be Macross tier with the potential for more than 400 Fox-3’s to be in the air, refreshing every 40 seconds).