A sports bag should be chosen based on the sport before the size: a racquet bag does not have the shape of a duffel bag, and a drawstring bag does not replace a club bag.
The shape of a sports bag comes from what it needs to hold. A holdall can fit an outfit and a pair of shoes. A bag for rackets is long and flat. A string bag carries just a swimsuit. Choosing purely by volume often leads to an unsuitable bag.
Four formats meet the essential needs, and they are not interchangeable.
A roller sports bag becomes useful as soon as the load exceeds what you want to carry on one shoulder: club equipment, goalkeeper gear, a full outfit plus shoes. It weighs more empty and takes up more space, making it impractical for a simple round trip to the gym.
A small sports bag of twenty to thirty litres is enough for the gym: an outfit, a towel, a water bottle. The separate shoe compartment changes everything, preventing soles from mixing with clean clothes. A sport waist bag completes the set for keys and a phone during workouts.