I didn't realize that. I had always assumed that Battles of Yavin 4 and Hoth both involved the entire Rebellion at the time they happened.
It's something they're only really starting to get into now, mostly with Rebels and the things that have been intimated by the likes of Filloni & Hidalgo.
While the old EU treated what we saw in the movies as the *entire* rebellion, the new canon stuff is a lot more nuanced. From what I gather this all came from Lucas's ideas (shock! horror!) the beginnings of which we saw in TCW with the Onderon arc. You take that, the
deleted scene from RotS and the goings on with 'Rebels' and it's prequel novel and you get a rough idea as to how things grew.
'New Dawn' is the key as it shows Hera, prior to leading an active cell was basically a scout and a spy for an unnamed individual (presumed to be Bail Organa) which is how she first runs across Kanan. So you get the sense than things would have moved *very* slowly, with Bail, Mon Mothma and probably the likes of Chuchi & Meena Tills quietly feeling out soft spots where resentment for the Empire was high, but fear of it was not all consuming and quietly building a network of agents. From there they grow and seed independent cells, which began to come together to form larger units following Ezra's broadcast with Bail and others funnelling them resources and equipment while still operating inside the Imperial bureaucracy, until each group of cells become a cohesive military unit in their own right.
The operation on Yavin IV would be one such larger cohesive group, though others would have been out there in smaller numbers. some local militias & gurilla fighters, some idealistic activists, others probably little better than terrorists. From what's been inferred, some of them (Saw & Cham for example) still operating independently and possibly even refusing to get involved in a larger conflict beyond their own provincial concerns.
By Hoth it seems the rebellion is still in at least two separate groups: the ground forces protected by a few fighter squadrons and their various small fleets, spread out across the galaxy. It's not until Endor that we see the whole thing fully assembled with the majority (if not the entirety) of their forces committed in one strike, with the Mon Calamari making up the bulk of the ships of the line.