There are 3 Ways you can increase your inventory weight in The Witcher 3.
Equip the Zerrikannian saddlebag which gives +100 to your inventory capacity. This merchant in Novigrad sells the Zerrikannian saddlebag for 813 crowns (You need to do a quest related to the King of Beggars to be able to get in first though):
If you don’t have money, the merchant near the Saint Gregory’s Bridge marker on Novigrad sells Rugged Saddlebags (+70 to max weight) for 407 crowns.
In Skellige Isle, the merchant in Holmstein Port sells the Zerrikannian Saddlebags for 828 crowns. He also sells Mastercrafted Cavalry Saddle for 887 crowns which increases your horse stamina to 75 (highest).
Fiend decoction gives you +20 to your inventory capacity. So having Zerrikannian Saddlebags and Fiend Decotion, you would have 180 inventory capacity.
The third way is through a mod. I’m using the Over 9000 mod and it has been a life saver. At level 20+, I was crafting and upgrading to the Enhanced Armor sets, and my inventory was quickly filled. I wasn’t even able to keep some relic swords with me for later use. I was left with 150/160 inventory space and during my adventures it would fill up pretty fast. In Skellige Isle, there are a lot of Treasure Cache’s in the sea. If you’ve tried to venture forth, you’ll soon find your inventory at max (even with fiend decoction), especially with all the heavy armor items you can get/sell. It sucks to pause your adventures to find a marker and go to a merchant to sell your goods. The workaround I did was to drop the not-used-at-the-moment items in Hierarch Square marker in Novigrad. It was good for a while. However, in my mind the thought kept lingering that those loot bags would one day vanish without me knowing.
So I resulted in using this mod and it’s been awesome! This mod changes the weight limit by 9999 (you can edit it also). To install, just follow the instructions laid in the page. At first it did get me stuck in Steam though. Cloud syncing was stuck at 0% and was never moving. However, what I did was just to turn off the cloud syncing feature of steam and the mod worked afterwards.
Now my Witcher Adventures are never cut short due to inventory space reaching full capacity!