It all depends on design. That is what all this testing shows. Generally shorter cans will have less back pressure, but suffer at the muzzle. As Todd from DA states, if you are starting to go over 146db at the muzzle, your at the ear numbers are irrelevant because the splash-back from the muzzle will be 140db++. And some shorter designs try to make up for this with higher back pressure stacks for bolt guns.
So basically if you are looking for a can that is as short as possible while still being somewhat decent at not blowing out your ears rather quickly then you want a can that will do <140db at the ear (the lower the better) and <146db at the muzzle in conditions closest to those you will be using it in. Remember that ammo, weather conditions, hosts, etc. can all dramatically change suppressor performance. Also keep in mind using suppressors indoors (which is what most people who want short cans will be using them) means that you want <140db at the muzzle because inside a car/hallway/bedroom there will be a ton of sound reflection. If your goal is not to instantly cause some hearing damage that is.
Having those kind of requirements really narrows down the field of candidate quite a bit, and many of the shorter cans just do not cut it for <140db at the muzzle (some of the best are cans like the Dead Air Nomad might just squeak in). Now you can throw that out and just take whatever sound reduction you can get at the muzzle as it will be leagues better than no suppressor at all and prioritize OAL instead. That is what most folks do who do a lot of serious CQC and you see cans like the Dead Air Sandman K, Surefire Socom II Mini, YHM Turbo K, Brevis II Ultra, and so on specifically made for those customers.