How to correctly utilise AssetBundles - Massive memory usage, 4x more than Resource folder

I'm having a lot of trouble figuring out how I am meant to use asset bundles so they don't take gigabits of RAM. I am downloading my bundles with:

 UnityWebRequest request = UnityWebRequestAssetBundle.GetAssetBundle(url,version,0)
 AssetBundle bundle = DownloadHandlerAssetBundle.GetContent(request);

Then saving that bundle to a variable in a singleton for access during runtime: data_bundle = bundle; Then, when I need the images for my cards in the bundle, I load them all into a dictionary from the bundle like so:

 imageCache[imageName] = data_bundle .LoadAsset<Sprite>(imageName);

And then when a card needs an image take it from the imageCache.

This is obviously wrong, I did the same thing with the resource folder (Loading all images from resources to a cache which was used during runtime), and I am now using more than 4x the RAM. Why? - what is the correct method to use AssetBundles?