reducing file size of post thumbnail images no working

Hi I’m cleaning up my site and trying to make it run faster and I’m going through reducing the size of all of my images

something I noticed is my thumbnail images are the same file size as the original

for example one image was 50kb so I reduced the file size to 12kb in photoshop but still left it the same size and saved it as a .jpg

when I upload it through wordpress to become a thumbnail not only is it still 50kb put it is also in .png format

can anybody help me understand why?