Milwaukee has made different claims about battery size depending on the tool. When I bought their first M18 Fuel drill, they said that you'd get more torque from the double stack batteries vs the singles, and I can confirm that. When I bought their first M18 Fuel impact driver, the claimed that the torque would be the same with either, and I can confirm that too. I believe that the difference is that the bigger motors can make use of the higher currents delivered by the bigger batteries.
On a related note, I was driving a bunch of 1/4" x 3" lags the other day, and I simply assumed that the task was out of the reach of my M12, so I put on my hearing protection, and used the M18 Fuel impact driver, and it did a good job on the first batch. When my battery was running down, I reached for the M12 Surge with the tiny battery that's just an extension of the grip, and it held its own against the M18 performance until I ran that battery down. At that point, I grabbed the bigger double stack M12 battery I have and the M12 Surge absolutely kicked the *** of the M18! I couldn't believe the boost in performance.
... SteveReal world experience says that the 9.0 and 12.0 definitely work better on the bigger tools. I have a 3/4" drive and 1" drive impact and the smaller batteries are not as powerful.
I know on the fuel 7-1/4 Circe saw a depleted 9.0ah batt has more torque than a fresh 5.0ah batt
when I bought the hi-torque 1/2" impact it was a bare tool, tried the low profile battery and it wouldn't even fit into the impact - by that point I also had a larger M18 drill, the bigger battery worked in the impact just fine.

I have a few milwaukee M18's, started with their "homeowner" drill/impact kit with the lower profile batteries - when I bought the hi-torque 1/2" impact it was a bare tool, tried the low profile battery and it wouldn't even fit into the impact - by that point I also had a larger M18 drill, the bigger battery worked in the impact just fine.
My conclusion: Milwaukee did this intentionally (by changing the keying slightly), so they wouldn't get bad press when "joe homeowner" tried to use too small batteries then bitched because their serious impact was a "battery killer" - just a SWAG, based on 75 years of listening to un-informed people complain... Steve
If a tool can draw down the cells of a battery that fast it means the cells are total garbage. If you notice a difference between 9.0 and 12.0AH batteries it means the batteries are total garbage.
Milwaukee folk talk about the power difference with higher ampere hour batteries on the regular, and how often the batteries fail. Sounds to me like the motors Milwaukee uses to generate their power are overtaxing the battery cells with too much draw to make up for poor engineering in the tools themselves.
I can run my impacts/drills/saws down to one bar, and it's the same as if I was running it fully charged. No power difference until the battery is just about dead.
If a tool can draw down the cells of a battery that fast it means the cells are total garbage. If you notice a difference between 9.0 and 12.0AH batteries it means the batteries are total garbage.
Milwaukee folk talk about the power difference with higher ampere hour batteries on the regular, and how often the batteries fail. Sounds to me like the motors Milwaukee uses to generate their power are overtaxing the battery cells with too much draw to make up for poor engineering in the tools themselves.
I can run my impacts/drills/saws down to one bar, and it's the same as if I was running it fully charged. No power difference until the battery is just about dead.
An exception to this is the 12V screwdriver. That has a clutch that's really light on the lowest setting and adjusts well. All my drills just stay on 'drill' mode.Most of the Milwaukee stuff stays in the wood shop, 'cause DeWalt didn't know how to make a decent clutch - all their choices are either heavy, REALLY heavy and "holy ****", aka locked up - both the compact and full size milwaukee drills could almost drive a #4 screw in balsa wood without stripping, and each click actually MEANS something.