I don't remember much about terraria... how many slots do you have, and how many items can be stacked in one slot, and what's the item with the heaviest real-world weight?
In terms of real materials, Platnum. Blocks in the game are 2x2 ft (assuming another 2, but its not 3d). Cant use ore blocks though so you would have to go with normal ore.
Each slot can hold 999 blocks and has 50 inventoryu slots. that equals 49950 pieces of Platnum at 2x2x2ft each. which is 8 cubic ft of platnum each.
Convert that to cm and each one is 2.2653e+5cm³
multiply 226530 and 49950 to get the total cm
3, which is 11,315,173,500cm
3now for the fun part. Since the density of platnum is 21.45 g/cm3, you take 21.45 and multiple THAT with the above, which becomes 242,710,471,575g
Convert to lbs and you get 535,084,996.19lbs with a full inventory, not counting ammo, armor, or coins.
Ofc my calculation are probably way off but even if the ore wasnt pure or if it wasnt fully 2x2x2 due to roughness it still is a huge amount of weight.