I like big bags and and cannot lie....
Ok, yeah, I dated myself with that "obscure" reference 😜
BUT, there are some bags of larger size that can be made even bigger. These are tradeskill made bags with names starting "Unexpanded Tailored ..." such as the Unexpanded Tailored Flawless Haversack, Unexpanded Tailored Transcendent Pack, etc. The only difference in the ones with the same 3rd name is an appearance. The 3rd name somewhat indicates the 'era' of the bag, the size unexpanded and expanded, and the materials used to make the unexpanded version.
When the bag is in an unexpanded state, it's tradeable. However, once you expand the bag, they're No Trade/Heirloom (even on Firiona Vie or other Free Trade servers) so make sure they're on a character/account you want them.
So, how do you expand the unexpanded tailored bag?
First you need to read a recipe book.
In Plane of Knowledge on the same platform as the Small Bank but in the building on the other side, look for the door behind the troll alchemist.
Behind this door is the book seller, Scribe Zikett.
If you have an Unexpanded Tailored Transcendent [bag], search through his wares for "bag" and buy the book.
If you have any other Unexpanded Tailored [something] [nag], search through his wares for "luggage" and buy the book.
Open the book and Scribe it by clicking the button.
Don't worry about the rest of books he sells... for now.
Now go to the other side to the building with the small bank and look for the tailor kitty and search his wares for "solution".
This is where you need to make a decision because the bag you have can expand into one of two bags, equal in size but different in weight and use.
If you want a bag to carry with you, with weight reduction, choose the expensive Abu'Kar Arcane Solution.
If you want a bag to put in your bank, choose the cheaper Inverse Arcane Solution. You cannot carry this bag in your normal inventory slots. But, when it's empty you can waddle over the the banker around the corner and leave it in the bank.
So, now that you have solution and bag, you'll need to combine them. This is why you read the book of recipes.
First, make sure the bag you want to expand is in a bag in your inventory, not in a base inventory slot. If you can use it as a bag, it is not in the right place. (I am guessing this is to make sure you don't have anything in the bag while it is converted.)
To your left is a loom. Click on it to open it.
Since you read the book, you can do a search on the recipe. Use part of the 3rd or 4th name of the bag, i.e. "tran", "ruck", "haver", etc.
Do not freak out when you see the skill required for the Unexpanded, because you have that, you're not making it. You want the recipe for the Light or Heavy (depending on the solution you bought) which is essentially a "no fail".
When you select the recipe, the ingredients will be displayed. Because you have the bag and the solution, these should both be green. If you have more than the number of items required, it will show you the number you have. If a recipe requires more than one of an item, the display will show each required item in a separate box. But, expanding a bag is just a bag and a solution.
Click Combine. Ta-da, you now have a bigger bag.
NOTE: These are not the largest bags available in game. But they are larger bags that can be made with tradeskills, thus no DB cash or
quest or achievement chasing. Another character can make the bag for you, you just have to expand it.