I'm pretty sure you're just being too careful. I think that the postal workers just automatically go to the lockers with packages because it would be a pain to try it in the small box, then have to go over to the locker, and return to your box with the key. So it's automatic.

I have a small box in one of those community boxes at home. I've noticed that one mail carrier always uses the big locker for a CD or DVD, while another one always shoves it into the box. Unfortunately, things that fit from their side do not come out on the front side as easily.

It must be a matter of personal preference.

Take Care,
Toky