Infernape is 3'11ft, so some starters can be on the smaller side. And Rowlet3 really dos not look too tall imo.
Yeah, Ambipom's got the same height, but given that both are chimps and not a gorilla or something, that still makes a ton of sense. But then again, Passimian (a lemur) is 6'07"... While Oranguru sits more or less in between at 4'11". The fully evolved Simi's and Primeape are all shorter then Infernape/Ambipom at 3'03", with Simisage being a bit longer because of the tuft, but still shorter then Infernape. Darmanitan is 4'03"..
I just realized that while still a sloth, partially gorilla-based Slaking is 6'07" just like Passimian. Point being that sizes can be deceiving, but monkeys
tend to be short.
Now... About those deceiving sizes.. Who thought that after seeing the animé decades ago that an ordinary Noctowl actually measures up to 5'3" (1m60).. Seeing how GameFreak has been pretty much consistent with their Simian sizes, Lemurs and Sloths not withstanding. With the Oranguru being the larger exception, the fully evolved ones basically sit around 3'03" to 4'03", which is just a foot difference.
And if you do count Oranguru, then it's just shy of two feet difference, but seeing as how an orangutan is generally seen as being a big ape, like a gorilla and not like a chimp, the size difference is understandable. From what we can tell, such a difference isn't apparent between the designs of Noctowl and Rowlet3, so we could infer that Rowlet3 isn't Infernape-like small. But rather in the general range in which most starters tend to fall. Which is around 4'11" to 5'7", some smaller some larger depending on their design..
I wouldn't be surprised to see if Rowlet3 is somewhere in between those figures, at all.