While Celebrity is a big cruise line, their Galapagos ships are not massive ocean cruise ships. I don't believe you'll come across any Galapagos cruise ships that carry more than 100 passengers, Celebrity ships included (and one of their ships is a 50 passenger yacht). If I'm not mistaken, larger ships aren't even permitted by law. So when you're talking large vs. small, you're talking maximum 100 passengers down to mid-range 25-50, and the smallest ones taking no more than 20 passengers.
Some people might choose larger ships for a variety of reasons, and how prone one might be to sea sickness is one of them (the thinking being a smaller boat will bounce and sway around more, while larger ones might insulate you a bit more from the movement of the sea).
Even on larger ones, the islands have their own rules that limit group sizes - just because you're on a 100 person boat doesn't mean all 100 of you will be trekking around together in the same group, with the same naturalist (not to be confused with naturist!) and guide. When I went on the MV Santa Cruz many years ago, there were about 80 passengers on board but also several different naturalists. All the passengers were sorted in to 5 different groups, each group with its own naturalist. When we'd arrive at a specific island, all the groups would gather by group and board inflatable tenders that'd then take ya to the island for a wet landing (which is to say, there were almost never docks or anything of that sort, and whether you were on a small boat or large boat you'd have to take a tender from your cruise ship or yacht to the shore itself, then hop out in to the water and walk ashore). So in those terms of actual landings on and walks around the islands, the experience wasn't that different - we spent all our time on land with our specific group, and each group would venture off in its own direction when on the island or stagger and alternate schedules so we wouldn't come across one another. More than anything, minimizing environmental impact was the driving factor on how landings are carried out. When I visited a couple years later on a much smaller boat, it was somewhat similar in that everyone on the boat was in the same group and so we'd all go on walks and island excursions together - same group size.
Certainly, though, there are certain islands and locations the larger ships can't go to - they can't get close enough to shore and are just too big for certain spots. Mind you, there can sometimes be some give and take here - the vast majority of live-on boats operating in the Galapagos are < 20 passengers, with only a handful of larger ones. If you're pulling up to a smaller island with a couple other small boats, you'll be seeing just as many other human heads as the people on the one midsize boat that happens to be at another island by itself. Overall, though, the limited number of boats allowed to operate in the area all do a fairly decent job coordinating things (not just for the sake of passengers, but impact on the islands and wildlife most importantly). When I visited on a smaller boat, there were occasionally a couple other smaller boats tendering passengers at a certain spot at the same time but it never felt remotely crowded or anything.
Speaking of impact on the islands, that is definitely something worth keeping in mind - your smaller boats will burn less fuel, generate less trash, and overall have a much smaller environmental impact per sailing. That's a worthwhile consideration, given how fragile the Galapagos are. If you want to experience them, while leaving as little of yourself behind as possible so to speak, there's something to be said for the smaller - and newer and more efficient - boat options available.