Otherwise, I'd say that even something like the Ep 5 -> Ep 6 fleet growth is fairly jarring.
I don't think so- There's a clear progression.
IV the Rebellion consists of a base and a bunch of snub fighters (so far as we are shown, though there's no limiting factor for branches of rebel cells.)
V opens with a new sort of fighter, but in the end we see that all those transports ships and it turns out they have a small fleet now. Although they are pushed out into space- the existence of small capitals ships- the Nebulon-B Frigate creates the feeling of progression.
Then once Han is rescued we find the Rebellion is snowballing support- even bigger capital ships, aliens are joining in large numbers (Both the Mon Calamari seen on screen and the Bothans referenced offscreen.) Plus the introduction of more civies in administrative roles- the Rebellion is bigger. But even still, it's not such a large fleet- I count four star cruisers, one or two frigates, a bunch of blockade runners and slew of those transports ships (for some reason). Plus the Original never made the mistake of saying 'this is all there is'- it's therefore easy to assume there are other branches of the Rebellion that are making moves and have consolidated everything together in a do or die mission.
TLJ made the mistake of limiting what the Resistance was.
"There are four hundred of us... on three ships. We're the very last of the Resistance. But we're not alone. In every corner of the galaxy... the downtrodden and oppressed know our symbol... and they put their hope in it."
So it's clear- this is the last. There's no other cells kicking around. No Procurement and Supply, no Reconnoiter, no other Resistance ships hiding around- these are the last- and by the end every single ship is destroyed other than the Falcon and they're left with a dozen. That's the whole Resistance
And those allies?
When the time comes- the Black Arrow is sent and Rohan does not ride. Helm's horn is sounded but Gandalf and Erkenbrand to not appear with 3000 on foot.
Instead "They've heard us, but no one's coming."
So to suddenly have a giant fleet makes zero sense. Destroying a third Death Star (not to mention the Death Star's attack in the first place) ought to have rallied the galaxy- but it didn't. Then blowing up the siege ship and Snoke's capital ship (and Snoke) ought to have rallied the galaxy- but even that couldn't convince them. Luke dying is an even worse reason for the galaxy to join. If they gave up before, there's no reason to join now when they didn't for reasons that one hundred times better. So it doesn't follow- magic fleet- but will probably have an unconvincing handwave. Quite frankly, by the end, I thought everyone sucked and just wanted the First Order to win. Cheering for Lawful Stupid isn't that much fun. Kill 'em and start over with a more competent rebellion.