"That 4" between 18" and 14" is a lot of lost space." And a lot of extra weight if you fill it up.
I flew BA RT Seattle to Heathrow last year. The bag I took is the standard 22x14x9. No one questioned it at the gate, but I tried putting it in the sizer near the check-in counter in Seattle. I managed to stuff it in, but I could barely get it out because the sizer had the frame on the inside instead of the outside.
Sneaky. It effectively made the sizer smaller. My bag sat on the bottom framing, not on the actual bottom. Parts of my bag got caught on the upper part of the frame when I removed it. A 22x18x10 bag would not have fit at all. If the sizer has the frame on the outside and the interior is totally smooth, 22x18x10 might fit because the square frame tubing won't be taking up space inside.
I expanded that bag to accommodate some purchases and checked it for the trip home. It's not such a big deal if I have baggage issues at home.
You might try Googling those dimensions or wide body carry on. Ever curious, I took a very quick look and it seemed like there were wheeled bags wider than 14," but not as wide as 18."