There was a thread a few weeks back on the actual size of the B&R bags. Nothing on the 18" bag but you can probably infer from the 20" differential. In a lot of other B&R threads there is the discussion over B&R carry ons. You do lose space with the outside handles versus a similar size carry on with internal handles. However, the trade off is the flat packing, which I like, but understand why others don't.

Quote from other thread:
Thank you for contacting Briggs & Riley Travelware. The external measurements for the following bags are:

U420LC 22.25”h x 15”w x 9”d
U420LX 22.25”h x 15”w x 9”d expands to 12”d
U420XW 22.25”h x 17”w x 9”d expands to 12”d
U420ZX 21.25”h x 14”w x 9”d expands to 12”d
U421LX 24.75”h x 15”w x 9.5”d expands to 12”d
U421X 23.5”h x 15.25”w x 9”d expands to 12”d