As it would appear some of this thread has gone bilge-ward, this hopefully won't get lost in the "banter".....
MPG - Miles Per Gallon. Not maybe the most useful way to describe fuel economy of a boat, as the water wil lmove underneath you, so for example mooring up a river against the current you wuill do a lot less MPG than if you have the river carrying you along. Other interesting tist t othat as a consumption figure is I think everyone (in europe anyway) buys fuel in litres, and o nthe water it's usually Nautical miles! Online conversion is a wonderful thing.........
As for your fuel consumption with an 18 Hp compared to a 25 and assuming a reasonably flat out trip, as a general rule the bigger it is, the more thisrsty it becomes. The leg / gearbox get bigger, so more drag, more weight etc etc....... At open throttle the bottom line is to get X HP you need to burn Y amount of fuel to get the energy to push the boat along. It's when you spend long times sat at idle / tickover (e.g fishing, dinghy rescue) where the 4- strokes come into their own. Also if you are in a sIB, the inflation of the keel etc will make a BIG difference to the drag - yours may have been pumped perfectly and your friend's keel a bit flat.
As for the engine on the transom, can't be of any help as any SIB I've had the engine comes off & goes in the back of the car so I can chuck the boat on the roof rack.