My Photoshop files are too large to make sense

I have been making a set of banners for use on twitter, twitch and youtube. I had absolutely no problems saving my version of a banner for twitter with it nicely downsizing to just 730kb, but the other two which use the exact same files on the twitter banner are still over 12mb! I honestly am so confused, below is the requirements for each of the two problematic files. Saving using both png and jpeg results in the same file size, decreasing save quality when saving as a jpeg has had no effect on file size and when saving for png using the "slowest saving" option only reduces the file size by about 3mb. I have been using a gfx pack dragging reference images in and out of the twitch and youtube versions but they are always deleted, that is my only lead as to how they are so large by comparison. Any form of help or input as to how I can fix this whilst still keeping dimensions listed below (minimum dimensions to upload the file).

Twitch:

Smallest Accepted Dimensions: 1920x480

Youtube:

Smallest Accepted Dimensions: 2560x1440