Your little darling has taught YOU how to FETCH!
Harley did this, and at first I thought he was just being stupid with balls, loosing them. Then I thought like you, give him bigger toys... oh no he wedges those under too.
What I do, is I clap or growl "no" when he's digging for the toy, I get the toy, and say "NO!" and I make sure he watches me put the toy he wants on a high shelf. I do not take it back down until he is out of the room.
I found that he connected the reason he lost the toy, if I showed him I was getting it from under the furniture, and showed him it, while I'll said no and put it away. He now only does this about once a week, and he used to do this about 5 times an hour.
The first week we did this, there was a pile of about 20 toys on top of my shelf, but I remained consistent and the behaviour decreased.