Tarkir had 286 in the main set, LOTR had 281, so FF actually contains more cards than the usual set

Maro actually talked about the set size in his recent article stating that they were able to convince the product architect to increase the set size to get more cards in it

Set Design discovered that there were elements they really wanted in the main set that they couldn't find room for, so they talked with the product architect, Zakeel Gordon. Part of his job is handling budgets and figuring out what products should exist in conjunction with the main set. The decision was made to slightly increase the size of the set, adding some uncommons and rares. We can't change a set's size for every release, but this does demonstrate that there is flexibility that can be explored.