One issue of the standard AirPods is that they adopt a one-size-fits-all approach with an in-ear design that does not always suit all ears adequately. If the fit isn't secure, the AirPods will easily fall out of your ears. The AirPods Pro address this shortcoming by adopting an in-canal design with three sets of replacement tips for small, medium, and large ear canal sizes.
Take an Ear Tip Fit test to find which of the three size tips will provide the best seal and sound. Attach one pair of the tips to the base of each AirPods Pro. Then, put both AirPods into your ears. Go to Settings > Bluetooth > [Your AirPods info icon] > Ear Tip Fit Test on your iPhone.
Continue to the screen to test the fit of your ear tips, then touch the Play button. A little music clip is played. Following that, the app will inform you if your existing tips are a good match. If not, it will suggest that you tweak your tips or try a new size. Continue through all three sizes until the app indicates that both AirPods have received the Good Seal of Approval.