It is indeed an interesting observation. Especially since I'm used to thinking of NanoBots as the fewest, since they're always calculated as a strict subset.
But I'm not sure I see the benefit of excluding smaller bots from the bigger rumbles? How would it stimulate interest? It kind of makes sense Nanos and Megas would be the most popular. If squeezing bots down to tiny sizes interests you, NanoBots are kind of the optimal version of that. If you're not, you might stick to MegaBots. I'm not sure why it's worth Wallaby losing his MiniMeleeRumble title, or LittleBlackBook not being allowed to take pride in crushing most MicroBots.