Please let the Square Enix is stingy narrative die. I was going to do a big breakdown of this, but I thought Sakurai's discussion on the Dragon Quest thoroughly dispelled the notions once and for all, and figured there was no point... That might need to be revived though...
I'll just point this out though. Street Fighter only got 6 songs when it debuted in Smash 4 and they were all just Ryu and Ken themes. Two were remixes, but the point is Street Fighter actually didn't come with all that much content in Smash 4 to begin with and Smash 4 DLC was clearly considerably more low budget than our current DLC for example. Final Fantasy VII music is notoriously more expensive due to Uematsu's licensing fees, so 2 songs at the time kind of makes some sense in the context of they barely had a budget for these bigger third party inclusions during Smash 4.
That Street Fighter content balloons when you get to Ultimate because they specifically added another Street Fighter character to the game. Konami and Capcom got the overwhelming amount of love for additions in Ultimate because they got characters, whereas the other three third parties didn't really get much. Sega got the most with 3 assist trophies (Rodan, Knuckles, and Akira... two of which were probably really easy to implement and license) and a couple of new Sonic songs (not remixes mind you). Square Enix added a Mallow spirit to its ranks. And Bandai-Namco picked up a couple remixes of songs... but lost a stage and a whole bunch of trophies did not become Spirits either, so they probably had an overall net loss. Not coincidentally those three companies were the ones who did not receive a new character in base Ultimate.
Or it could be that SF got additional content because it was no longer hindered by the budgetary restraints of 4's DLC. All it got was treatment tantamount to any other base addition, non-retro series, being a fairly substantial amount of content across the different forms of representation.
It came from a deficit to the norm. The only other series working from a deficit was, really, FF. Bayonetta, being a series presumably cheaper and easier to license, already came with a fair amount of content the first time. Though it still got Rodin.
What this is indicative of is an additional supply of capital for Ultimate. Sakurai simply brought the series to par with the standard given he now had the funds to do so. I'd think (well, hope) you'd agree that it'd be ridiculous to suggest Sakurai had no ambition to expand the absolutely paltry stable of FF content simply because no one was coming to join Cloud. It's certainly not Sakurai's MO to coast by on the bare minimum. It'd be a different story if FF didn't receive much new content yet already had a lot present.
The roadblock had to be somewhere at Square's end. Whether they asked for more money than Nintendo was willing to spend to flesh out the ranks of the series, or they had inexplicable regulations by which Sakurai had to capitulate, the culprit for the lack of parity is not the company or director who has in all other cases made the effort/investment to keep things relatively level.
So use whatever term you want, say SE aren't stingy, it seems evident that, in certain areas, they either just only respond to being "compensated handsomely" or they are "very particular about representation". Possibly both.