and, instead, just switching the labels

The labels may also be part of it. Vitamins need a lot of mandatory nutritional information printed on them, and they may simply have been unable to print all of that (in a reasonable font size) on a smaller bottle.

---

But, of course, the real unspoken reason is that it’s deceptive marketing. People who don’t read the label carefully will see a bigger bottle and think that it means they’re getting more product inside, even if a competing brand’s smaller bottle actually contains more.