There is a cap to the amount of loot that a raider can take from your storages, but it's not affected by the level of the storage in question. You can find the exact formula on the CoC Wikia page for Raids.
For instance, if both of you are at TH8, the enemy can take 14% of the gold and/or elixir in your storages. If you had 2.1 million or more in your gold or elixir storages, the cap would kick in and the max they could get would be 300k of each.
The only way the number and size of your storages factors in is in how those resources are distributed and how easy they are to take.
In my experience, troops hitting a storage take loot linearly. For instance, if Archers hit one storage and reduce its HP by half, you'd get half of the available loot from that storage.
By increasing the HP and/or distributing the loot among multiple storages, you reduce the amount that can be taken each time an attacking troop hits the storage. This gives you more time for your defenses to kill raiders before they get everything that they could from your base.