Sorry, I meant the fleet as a whole. The purgill wrecked a few ISDs and another was crashed at Scarrif, but that's a paltry few ships in the Imperial Navy. Definitely not enough to hamper operations, and in regular engagements with rebels (the usual, small skirmishes with a squadron of small rebel ships) during the entirerty of the rebellion, I can't imagine many Imperial ships were damaged or destroyed, but would certainly be tied up prosecuting those battles, and, after Yavin, chasing down the rebels was top priority.
given the success at Scarif and Yavin*, and word getting otu about Lothal, it could be that you saw a major upswing in worlds actively opposing the Empire via insurgencies or outright uprisings. the imperial fleet may be huge, but the Galaxy they are trying to control is huger still.
*one of the reasons i've put forward for why Scarif and Yavin are the victories that kickstarted the serious rebellion, and not lothal, is how the Empire would have to respond to Scarfi and Yavin. basically, with Lothal they could pretend for the wider galaxy that it didn't happen. lothal didn't have much of a trade presence before the empire came, according to the supplementary materials, so the Empire could claim they relocated the factories and withdrew the garrison.. basically pretend the liberation didn't happen. at least for as long as they'd need to arrange some sort of real response (which got derailed by Scarif-Yavin and what followed)
At Scarif a major Imperial facility, a hub for nearly all data traffic through that half of the galaxy, was destroyed. and since the Empire cannot claim that it was an accident. and wouldn't want to admit Tarkin blew up their own facility for largely petty reasons, they would
have to blame it on the rebel alliance, which had a fleet there.. but in the process they'd have to admit that not only did the rebels have a "great victory" but also that the rebellion is much larger, better equipped, and more organized than their propaganda had been telling the people. it was a lose-lose situation. and in the days that followed, you had Alderaan blown up by the Death Star as part of a demonstration about how powerful the empire is and to intimidate worlds into toeing the Imperial line.. an act that not only requires them to publicly admit they destroyed the world but also reveals that a weapon capable of destroying planets is in imperial hands. this would further undercut the propaganda the Empire uses to sell its totalitarianism to the people as being in their best interest. alternately, if they try to claim someone else built and used it, it makes the empire look weak and ineffective at protecting people. (geeze, tarkin is good at getting himself into those isn't he?) again ,a lose-lose situation. and no one is going to take the idea of the rebels building such a thing seriously, since there would only be one feasible group withthe resources to build such a thing. then cam Yavin, where the Rebel Alliance managed to destroy said superweapon. while the Empire would not be about to advertise that aspect, the fact is that the Rebellion would be.. and combined with the arrest of the senate and the other actions the Empire took in those few days, would make the rebel message very potent.
so basically, Scarif-Yavin is the pivotal moments because it put the Empire into a situation where no matter what it did.. the Empire came out looking terrible, and the Rebellion looking pretty good. Lothal on the other hand is a fairly minor side event.. the Rebel Alliance proper wasn't even involved, and if not for the timing, the Empire easily would have just steamrollered the world back under, or worse, rendered it uninhabitable with a "Base Delta Zero Initiative" (which given we had reference to that being used to 'liberate' 'another' world earlier i nthe show, this suggests that worlds rising up and kicking the empire off was fairly common.. it just usually didn't last.)
