The original script describes them thusly;
There are no windows, but there are a few doors leading to other parts
of the building. The doors are low and tunnel-like and even Ferengi
have to stoop to get through them. Rain from Ferenginar's relentless
downpour can be heard beating against the roof.
Quark and Rom ENTER, followed by Brunt. Each Ferengi deposits a coin
in a collection box by the door, then removes a towel from a rack, and
begins blotting off the rain.
The fact that they look like tunnel entrances may be a nod to the fact that the Ferengi are more animalistic, living in literal dens that have been modernised.
I've seen web-chatter that one of the EU novels mentions that the doorways became smaller over time as the Ferengi realised that they were among the shortest of the Alpha quadrant races, as a form of compensation for their obvious inferiority.
I liked the explanation offered in one of the books that Ferengi
doors are too short for them. After gaining spaceflight and realizing
they are shorter than other humanoids, the trend for small doors
emerged as a way to make them feel taller in the same spaces. I kind
of retroactively assumed that's what the designers were actually