Every item has a Size ⇲ and Weight ⚖. An item's size and weight is shown on the tooltip, which appears when you hover over it with your mouse.
Size determines what storage blocks an item can fit inside of.
Weight determines the max stack size of items.
Most items are Very Light by default. Blocks are usually Medium.
Overburdening happens when you carry items that are both Huge and Very Heavy. Carrying just one Huge, Very Heavy item causes you to become exhausted, making your food burn quicker. Carrying two or more gives you the Overburdened status effect, which makes movement very slow.
Items that count towards overburdening include Anvils, Sealed Barrels, Crucibles (with contents)
Note: Horses can also be overburdened.