I wouldn't worry about Lemmy having as many users as reddit in the short term. Success is not just a measure of userbase. A system just needs a critical mass, a minimum number of users, to be self-perpetuating. For a reddit post that has 10k comments, most normal people only read a few dozen comments anyways. You could have half the comments on that post, and frankly the quality might go up, not down. (That said, there are many communities below that minimum critical mass at the moment.)
Lemmy is now a real alternative. When reddit imploded Lemmy wasn't fully set up to take advantage of the exodus, so a lot of users came over to the fediverse and gave up right away. There were no phone apps, the user interface was rudimentary, and communities weren't yet alive. Next time reddit screws up in a high profile way, and they will screw up, the fediverse will be ready.
Lemmy has way more potential than reddit. Reddit's leadership has always been incompetent and slow at fixing problems. The fediverse has been very responsive to user feedback in comparison.
Honestly, I don't know if it's the fewer users, the lack of trolls, the newer apps I've been forced to use or the topics that I've been getting into since joining Lemmy. But I have been considerably more active here both commenting and posting, than I ever was on Reddit.
It may have started as a way to do my part for the growth of Lemmy, but it's not been about that for me for some time now.
For me it's the smaller number of users. It is very likely that your comment will just end up at the bottom and nobody will see it if you comment on a reddit post with thousands of comments. If you comment on a Lemmy post with 25 comments or less it is way more likely to actually be seen by people.
Others have touched on it, but for me it's like the difference between speaking up in a conversation between people I don't know at a house party, and speaking up in a giant auditorium when the person on stage is asking for inputs. The smaller scale makes it a bit more comfortable and I feel more like what I have to say isn't already being said by a hundred other people.
Totally agree although sometimes Reddit was a lot more like speaking up in a bar full of angry drunks right after a group of neonazis burst in and started slap fighting everyone.