It's just that it's much more nuanced than simply a large gym vs small gym. For instance I would call my DDs gym a 'large' gym based on the size of their facility and the number of rec students. They have two large gyms (toddler and big kid) plus two additional spaces that can be used for dance and/or other specialized instruction. The main gym has two competition floors, three vaults, pit, two tramps, several sets of bars and well over a dozen beams. They have both Xcel and DP. The DP team though is intentionally smaller and would qualify as a 'small' gym. Sometimes they have 2 L10s, sometimes 7 or 8. They don't take in many kids from other gyms so this is mostly based on the size of the original preteam group and how many stick with the sport into the upper levels. They send a kid to D3 every other year and a kid to D1 every 4 years or so. When my DD joined DP the head coach told me that their goal is not college gymnastics but rather an emphasis on safety and the well being of the athlete. In every preteam group there will be one who leaves for a gym that has a more solid track record of sending kids to D1 (usually these kids don't even make it through high school but the family wants a different experience where odds for NCAA are in theory higher).
Then of course, you are limited by what is even available in your area. Families in a major metro will have several options and will have to factor in how long of a commute they want. Some towns are lucky to have one facility.
So it really comes down to the gym that will provide the best experience for your child and family - and the specific qualities that are prioritized will be different for each family.