I came up with a workaround to stop ppt from re-sizing the image when imported as a custom bullet. I put a 10 pixel x 10 pixel custom bullet point image on a 20 pixel x 20 pixel white background. By doing that I was able to maintain the small image size of the custom bullet point jpeg.

Previously, I had been cropping the jpeg image close to the edges of the custom bullet point. Then when I was importing those images into ppt, the system kept scaling up the image size. Since ppt wanted to scale up the image to a certain size, I increased the overall image size with white background while putting my custom bullet point as a smaller element within that background.