I have to agree with Jess. There's no hard and fast rule about trainee intake size and how that impacts you - most 'good' firms would have thought carefully about how to strike the perfect ratio of trainees to lawyers. It's a cost and a talent pipeline consideration, and for them to get it wrong would naturally be costly (literally) and would hurt their deal/capacity structure.
People also are quick to forget that trainees are spread across different departments and you’re almost always, at least in my experience, the only trainee on all your deals/matters. The benefit of a large trainee intake is that you might have someone who can cover you at your level, without having your work 'move up' to the next most junior person. Large intakes also benefit you insofar as you have someone at your literal level to ask questions. I won't comment on the other 'benefits' or 'negatives' that surround making friends, social life, etc - since that's person-to-person dependent. From a networking perspective, it's an inherent advantage though (more people who may be potential clients).
From my perspective, I don't agree with the generalisation that "small trainee intakes firms are ‘better’ as your responsibility is greater". For a large trainee intake 'firm' to negatively impact your development, you would need a perfect storm of trainees in your practice group asking to be put on every deal (regardless of their capacity), poor resource allocation from the partners or managers, and a really quiet department for a large trainee intake to impact your responsibility level (and this is on the assumption you’re just a quiet person who doesn’t raise their hand). You can already see there's a lot of agency for yourself to determine your responsibility, more than other extraneous factors.