Are my expectations a bit high with this?

[Ozziegirly]

DS is 16 months. He eats in a booster seat at the table and does a pretty good job of it in general.

However, when he has finished, he carefully deposits the remains of (say) a cracker or a corn on the cob husk carefully onto the floor.

I reckon I have said every single meal for the best part of 8 months now "When you have finished, leave the food on the plate" and he will look at me with recognition, and then 3 minutes later, onto the floor it goes.

I leave him in the chair when he has finished while I pick the bits up off the floor, and say "You could be playing now, but I have to pick up what you dropped on the floor".

Am I just expecting way too much of him? He's so quick at picking other stuff up that I can't believe he doesn't understand!

This may really irritate you but he is very young. I would see this as "small stuff" and wouldn't sweat it. He won't still be doing it when he's 16 years!! It seems a very tiny piece of rebellion or, more likely maybe it is just his way of saying, 'Please may I get down now, I've finished?' Wait until he brings home an unsuitable girlfriend, fails his Maths, loses his mobile, stays out all night etc You will then have fond memories of when he dropped a piece of cracker on the floor!!

He's learning about gravity and showing you he's finished. Worrying about it is only going to drive you up the wall. How about you let him down when you think he's finished, rather than telling him not to throw, then he won't get the chance to throw?
DS is just coming out of a phase of throwing everything at 17 months, but he still will if I don't let him out early enough. He also still thinks being told not to do something is a hilarious joke, I'd be very surprised if a 16 month old actually understands about not messing up the floor, or why he has to sit and wait for you to clean up.