38mm Cap Feeders specification should start with the real closure.
Larger caps can need more space in the bowl, wider tracks and different chute support. The feeder should prevent tipping, overlapping and excessive cap pressure before the capping head.
For a better buying decision, the useful comparison is not only hopper size or headline speed. It is how the feeder behaves with your closure, how the capper receives the cap and how easy the line is to run after changeover.
The safest specification route is to start with real closures, target output, line photos and the exact point where the cap must be presented. That avoids choosing a feeder on a generic speed figure that does not match the cap or capper.