Its not necessarily about number of fights per division.
It has more to do infrastructure and support.
Imagine if you were a MMA trainer and your fee is a percentage cut of a fighter's earnings. Let's say an average welterweight in the UFC is paid an average of $100,000 a year through salary and sponsorships. By comparison lets say a featherweight might make $50,000 a year(I would guess it could well be less than that).
Whereas some fighters at LHW might make $250,000 per fight in salary a fighter at flyweight or featherweight might only be paid $8,000. The pay is massively smaller which provides less opportunities for fighters in lighter weight classes in terms of training camps, hiring support staff, etc.
There's a much lower influx of capital into the lighter classes which translates to fighters being less developed and further from being able to reach their peak potential.
The lighter weight classes at present are like the UFC in its earlier years when things were less developed and fighters were less well rounded. It used to be that fighters were for the most part only good in one area. They weren't being paid enough to be able to hire a jiu jitsu coach, a striking coach and a wrestling coach and train full time. There wasn't enough money available to them & in a lot of cases they may well have had to work 1 or 2 jobs and train in their time off which would have been difficult.
I think that's where the lighter classes are at now. They're not being paid a lot, they don't have the cash on hand or resources heavier weight classes have which would allow them to reach their peak potential. And the lighter weights don't have the depth where there is a large pool of talented guys who have been in the game for years and years. There are only maybe a few...
In a way comparing the older heavier weights with the newer lighter ones are like comparing MMA in its infancy to boxing in its prime. Of course, when boxers were making $20-50 million per fight & boxing is a developed sport with decades of history it makes sense that there would be the type of infrastructure and knowledge base that would allow fighters like Tyson to achieve a lot.
Whereas in MMA, when fighters weren't making a lot of money and there wasn't a lot of type of support or infrastructure and the sport is in its infancy it naturally follows that it would take time to develop.
I think that's how it is with the lighter classes, it'll take time for them to develop and how fast or slow it happens may be inversely proprtional to the necessity present in terms of monetary valuation. If flyweights are paid $500 per fight, that doesn't allow them to do what they need to do to train at their peak. Likewise considering the lighter weight fighters generally fall into that pay grade that doesn't leave them with a lot of options, I think it could be some time before they can afford a full training camp with the best coaches and nutritionists and support staff. That may well be the main obstacle that will prevent the lighter weight classes from being as deep, developed or skillful as the heavier classes which makes comparing them difficult.. maybe.