Chests are levelled, i think open ones create loot when you get within a certain range of them, locked ones are generated when unlocked. So yeah if you leave a locked chest for late game it will contain better loot.
This is true with the original ED, but I had some doubts for the DKS version. While still on the other patch 1.4.9.32, when I try collecting all my locked chest in broken valley and sentinel island before facing laiken, I found alot of locked chests in the starting area such as broken valley village, the contents seems not on par with my character level. Eg a rank 4 locked chest in the barracks, which I tried reloaded for quite some times, only seems to provide equipments lower than level 4.
And while comparing to the original ED which I still roughly remembered, there will always a piece of equipments in locked chest, be it armor or weapon. But in the DKS version, majority locked chest only provide me with minor gold and low rank potions. Maybe out of 10 relaod, I will only encounter equipment once as contents for majority of the locked chest.