I know what you mean. It depends on the string spacing. I'll take from a few frames of mine.
The Wilson Pro Staff Midsize is a 16x18 but it was advertised back in 1984 as having the strings centralized to minimize the trampoline effect and thus offer more control. I can compare it to another frame of mine, the Wilson Graphite Force midsize, an 85 sq. in. head without the PWS. It's a 16x19, but the strings are relatively evenly spaced and at the center of the string bed it's more "open" than the Pro Staff Midsize. Honestly, I think that's one of the contributing factors on why I prefer the Graphite Force over the Pro Staff.
Another frame is the Prince CTS Synergy DB 26 MP. I think it's a 16x18 on a 98 sq.in. head. There is quite a bit of space from the sides of the frame to the first mains as well as the top and bottom to the crosses. The center of the stringbed is quite dense. I think that translates to a firm response.
The 16 main vs the 18 main is at least a starting point for looking at string pattern as a specification, but I agree with you that it's the spacing (density) of the overall stringbed that is key.
"The secret is to find your motivation every single time - your inner motivation - so I always want to improve myself..." - EV