How do you stop items from being pushed through walls in Minecraft?

I am working on an automated chicken farm. Things are going quite well and my prototype is working, but I am having a small issue that affects my efficiency.

For my kill step, I have pistons open up a stream of water that carries the chicken to a piston smasher area. This area has a ceiling of sticky pistons which push dirt, and a floor of hoppers which collect and distribute dropped items.

My problem is that chicken meat and feathers keep popping out the top & bottom of my contraption. About 90% gets collected, but 10% ends up on the factory floor. I want to eliminate this waste.

How can I stop the items from getting pushed through my walls? Am I using the wrong block (all the non-essential blocks are dirt for easy construction)? Do I just need to make everything 2x thick? Am I missing something? Is this just how Minecraft operates?

edit I am doing this on a 1.7.4 vanilla minecraft server

edit 2 Heres an image of my killing area. I removed a wood slab so we could see inside a bit better. Note that this is the off positon, chickens aren't ready to go in yet. Directly under the dirt being pushed is a whole floor of hoppers

This is essentially how Minecraft operates: when items spawn inside your crushing block, the game tries to move them somewhere valid. Any non-solid block, like slabs or stairs, that won't push items, also won't crush chickens. Double thick walls may help, but not guaranteed. The game will just try to push items harder.

The solution is to put some kind of non-full-height block above the hoppers. This way, chickens will still have their heads suffocated by a block above, but their legs and dropped items will be in a valid space above a lower block's surface. Hoppers suck items in from the whole block space above them, so that would not be a problem.

Testing indicates that the obvious choice of short block - the Slab - works perfectly.

Also check the list of block heights here: http://minecraft.gamepedia.com/Solid_block#Heights

Eventually I fixed this by replacing the dirt on my pistons with soul sand. Soul sand is slightly shorter than a full block (7/8s). This leaves a small space between the hoppers and the smashing blocks, so that items are not forced thorough.