Hi,
MaterialMArt is totally responsive, the sizing of grids/pocutures won't be controlled by actual image sizes. Sizing of the pictures and the grids are depended on how many grids you have per row. If you choose 2-3 products or 2-3 categories per row, then the image size will be big. But if you choose 4 or 5 products/categories per row.. then the sizing will be relatively smaller.
Regards.