That only works if your FC leader actually allows the chest to be used in such a way - we had to lock our chest to stop people making off with expensive furnishings we were storing in there (only my bf the FC leader has the authority to remove items and gil from it, not even I have access to it and I agree with his decision). So that is not really an option for most players I'm afraid.

Personally in regards to inventory and retainer management I've gotten myself into a routine now of getting rid of all gear I know I can get again, especially low level vendor equipment which as I have all my Disciples of War and Magic above level 50 now, is no longer necessary (why bother keeping that junky level 22 sword I never used and never will now that my GLA/PLD is at 70?). If it is fully spiritbound, I convert it to materia (which I then sell on the MB), if the gear itself can be sold on the MB I sell it there (even if it's for 500 gil, 500 gil is still 500 gil), and if it's pink/green/blue gear that I have no use for or desire not to use for glamour purposes (and which I know I will be able to acquire again if I wish) it goes straight to the Adder's Nest for GC seals.

And it should go without saying that anything that can be stored in the Armorie is (indoor furnishings I have likewise put into the Storeroom as you'd be surprised just how much furniture you accumulate during the game, most of which I have no use for but can't bare to part with).

I only hold onto gear if I still have a use for it or if I know I can't get that gear easily again. Hence I have quite a bit of free space left between my inventory, armoury chest, 3 retainers and the chocobo saddlebags.

I think those who have no space left are those who hold onto crafting materials so that does explain why I have a bit of space left (I don't bother with crafting as it bores me to tears).