*this is just a suggestion based on preference not a statement on real life body types*
For those not looking to use mods but would rather avoid the muscular bulk it's most easily obtained using the endurance run option on a treadmill, only eating when the need is orange or doing salads a few days, and boxing. Another way to avoid muscular bulk is to juggle the soccer ball. Most of my sims are lean and tone or lean and muscular. I use the endurance run most often with some boxing and only push limits on endurance run. I've found it's the easiest to maintain and requires less time in the gym. They also regularly juggle the soccer ball on their own or with another sim when idle without me prompting.
Hitting the weight lifting machine, commando training, hill challenge will add both more "bulk" muscle if you're looking for more bulk to the frame. Eating more often in the green and push limits on the weight machine or hill challenge.