The thumbnail size depends on the size of the main image in the product page. You can find different designs in the Theme Settings > Product image > Images https://prnt.sc/pzoyyr
The thumbnails would not 150×150 but they will be bigger. The settings 150px exactly would require complicated customization which is not covered by our support, consider options in the Theme Settings > Product image > Images, consider the placing thumbnails below the image.
The size of the thumbnails when place below the main image is more acceptable but having them to the left of the main image is a much better layout and user experience.