One Iron Chest holds the same number of stacks as two wooden chest, however you cannot place two Iron Chests together and have them become a connected inventory.  This means that 1 Iron Chest is equal to 2 Wooden Chests placed next to each other.  

The chests can be upgraded using craftable upgrades. They can be used by firstly you having them in your hand, then you must face the chest you would like to upgrade and then you right-click and the chest will be upgraded!

The upgrades go as follows:

Copper ------> Iron ------> Silver ------> Gold ------> Diamond ------> Crystal OR Obsidian

Both the Crystal and Obsidian have an special ability.

The Crystal chest lets you see what you have stored in the chest. As it is see through it lets you see some of the items stored in the chest. It also has the storage of 2 Diamond Chests. 

The Obsidian chest lets you keep your items very safe as it is resistant to any type of explosive damage meaing that it will not break when in the radius of an explosion. It also has the storage of 2 Diamond Chests.