It seems these intelligent slimes have gotten a hold of Felix, and have began feeding themselves to the confused lad. As he had just been there to deliver some packages.

Though why is this. Slimes in general tend to indulge more, and end up quite large. They sometimes share their slime with smaller ones to help them get larger. So when Felix just so happened to deliver whatever it was they ordered. They saw his poor small (which in reality was fairly average sized) frame and decided to help fix that as a tip.