Why does my brake feel loose?

About a month ago I changed my front brake disc and pads, along with flushing the brake system. Which turned out perfectly fine :). Then today I decided to do my rear one. To my behold it was too hard because they were drums, so I just did only one side but the differences in the the horseshoes in comparison of my old to the new ones was barely one cm(the reason why I wanted to do it was to learn). So after I've assemble the new drums on ONE rear side I expected the brake pedals to be loose. So I drove for awhile and yet it is still loose because I'm having to push the brake pedals further down. What causes this?