Nagrand (64.7, 35.7) under the bridge.

The important thing to note here is that the bag is not unique and not soulbound. It is the largest bag of it's kind by far.

Could you summon a low level character with a warlock portal to Nagrand and farm this bag on alts? Or is summoning/looting treasures restricted?

That's ogres for you. Heads the size of 霜纹包, brains the size of 小钱包.

With 6.1, they made this BoP. You used to be able to mail this to other characters.

Who the hell needs a 20 slot bag at level 98? >:C

In case you wonder where else to get large bags for little effort:

Questline: ~ 20 minutes
土地精包裹 (24 slot) from a questline starting with 交通问题 in 昆莱山 Mists of Pandaria

Raids: each ~ 2 minutes once you are there. Soloable level 90+ and 100% drops
龙皮包 (22 slot) from 黑曜石圣殿 in 龙骨荒野 Entrance under the Wyrmrest Temple
深渊领主的口袋 (20 slot) from 玛瑟里顿的巢穴 in 地狱火半岛 Entrance bottom of the wall in the center of the map, eastern side
加大的奥妮克希亚皮袋 (22 slot) from 奥妮克希亚 who can be found in the raid in 尘泥沼泽

Dungeons: ~ 10 minutes
破烂的巫术布包 (24 slot) from 妖术领主的神像 Zul'Aman Dungeon 85 heroic

Treasure Item:
食人魔潜水头盔 (20 slot) in 纳格兰 Warlords of Draenor

When you swim under the bridge, there is a skeleton and the bad is hard to see but its behind it

As of Shadowlands requires level 25 to loot

Currently requires level 35 to loot.

小知识

屏幕截图

视频

食人魔潜水头盔

指南