Thanks for the suggestion. I tried importing an EPS file, but it didn't make a difference. However, I was able to fix the problem by enlarging the image in Illustrator, not Powerpoint. After a series of more tests, here's the conclusion I came to:

When a vector is imported into a Powerpoint document on a Mac, it will stay in lossless vector format on other Macs. However, if the same document is opened on Windows, the vector information is lost, and the same image is rasterized at its original dimensions. Weird.