There was the advantage of greatly reduced rocking couples with inside cylinders as compared to equivalent outside cylinders.
"...spoiling the outline"??? I bet you're one of those philistines who thinks locomotives should be rectangular boxes filled with nasty Dieasal stuff and distinguished only by hastly repainted numbers with a computer check code appended(?)
Well, they were (generally) cylindrical in shape and often about the right size for their intended tasks. Of course, a _proper_ steam loco should have full valve gear with lots of extra rods and levers that only the designer understood while he was drawing them on his plan.
Regards, Greg.P.