The medium size earbud worked fine from the outset, but we are encouraged to try out the others. I soon discovered that fit has less to do with the diameter of the bud inserted in the ear canal than with the offset from the canal of that portion of the outer ear called the concha. More specifically the tip of the flexible arm must lock into the narrow apex of the concha called the "cymba." Anatomy texts don't always describe the cymba, but all patents for these tips do! Neither the small nor the large came anywhere near the intended target. That's why ill sized earbuds fall out.
After all this fiddling and upon restoration of the medium tips, I got a lot of squealing (feedback). Feedback was mimimal only when I followed the exact instructions for reinstalling the tips that specify rotation.
Query: why don't the large tips have the white inner plastic retainers found in the medium and the small?