The leaves will decompose pretty darned quickly in those conditions. Then you'll be left with what could be sludgy mess in the bottom of your container..and your plants sitting several inches lower than you intended.

You need to use something that won't disappear. Some people use empty plastic bottles or cans. I've used those webbed onion and potato bags, filled with styrofoam peanuts. I really like how that works!

Wouldn't it be better to just use a smaller container?

What's the goal, to increase the amount of potting medium you have by adding leaves or decreasing the volume of potting soil in a container by filling up the bottom? If it's the first, why not just mix it all together? If it's the latter, why not just use a brick?

What's the goal, to increase the amount of potting medium you have by adding leaves or decreasing the volume of potting soil in a container by filling up the bottom? If it's the first, why not just mix it all together? If it's the latter, why not just use a brick?