I took the opportunity to measure the two kinds of firelogs. One of the few North Idaho logs we left is at the top: it's 32.5 cm long and has a diameter of 10 cm, which is about 2,550 cm3. One of the Pres-To-Logs is at the bottom: it's 29 cm long with a diameter of 9 cm, for a volume of about 1,840 cm3. Both are made by similar processes of squeezing sawdust until it forms a log shape. That's why they both dissolve in water; there's nothing holding them together other than the initial pressure under which they were formed.
The North Idaho logs are about 140% the size of the Pres-To-Logs, but the pallets are roughly the same total size and weight, and they seem to contain roughly the same calorific content, so they are broadly comparable. However, I do prefer the larger logs and would buy them if it were easier to get them.