In The Recollections of Rifleman Harris, they walked, everywhere. I mean miles and miles and miles. All the time. That kind of constant movement will keep you trim. For example, when Harris enlisted, he walked to where his training and post were, then walked to port to take a boat to Ireland where he walked everywhere. Then they took a boat back and walked to wherever they needed to embark for their campaign. When they arrived, how did they get around? They walked. The book details the retreat in Portugal. People walked so much they would die regularly. It should also be stated that many soldiers were farmers, shepherds etc. so they were already leading fairly active lives.

There was also drill, where soldiers would learn and practice hand to hand combat, how to march and perform battlefield and parade maneuvers, all of which take energy.

You have to think of enlisted men of the 18th century almost as marathon runners, often doing lots of muscle intensive activity on insufficient caloric intake.

Marching: Take simply daily marches on campaign for a soldier in the American revolution: anywhere from 5 to 20 miles a day, marching. Their kits are anywhere from 40-80 pounds, depending on gear, season, weaponry, food rations. This isn't necessarily on nice, even roads. This is through tough terrain, in bad weather, through valley and over mountain. During battle these troops had to move, sometimes in full kit, quite quickly. At the Battle of Brandywine some soldiers move nearly four miles in full kit in about an hour. A decent representation of distances over time can be seen in this animated map of the Philadelphia campaign: http://www.westernheritagemapping.org/PhilCampaignAnimated/PhilCampaignV204.html

Labor: A soldier out of campaign, say in camp or encampment isn't going to be sitting around either. During the winter encampments, they have to build fortifications, get water, build huts. In many of these encampments wood is not near by due to the land being farm land or already cleared. Soldiers thus must travel miles for trees. Each hut takes anywhere between 20 to 80 trees to build, trees are a mile or two away. Days would be spent hauling trees miles, cutting them up, notching them out, putting them together for a hut structure. Fortifications need such wood, but also require substantial digging and hauling of earth. See: Valley Forge, Morristown, New Windsor Encampments

Training: Add in days of training, sometimes 4-8 hours per day of training days during winter encampments.

Add into that soldier's rations were supposed to be about 5000 calories a day, but often sit under that. Soldiers are often running a calorie deficit and thus are going to be lean and muscular. https://research.archives.gov/id/5711548

This is one of my major pet peeves as a historian who often interacts with the world of reenactment. Reenactment is an older man's game, since it requires so much money. Thus reenactors tend to be older, heavier and not representative of the young, sinewy, lean, soldiers that fought our revolution. Some groups do this well, and I know many personally who are doing great work towards getting away from this, but it is an uphill struggle.