What are the sources and/or studies that justify this claim?
Unlike long-term memory, working memory has a limited capacity and duration. This limits the number of schema (information) that can be managed at one time. CLT was originally designed to describe the interconnected-ness of worked examples when it comes to learning. Visual elements should have context, otherwise you are imposing more things for the mind to process (for example, an unnecessary puppy on a PowerPoint would require the mind to process its a puppy, determine it is not relevant to the information, and continue to ignore it). Long-term is uneffected by this process, but the stress imposed in working memory requires effort to maintain.
To answer the question of "What are the sources and/or studies that justify this claim?" - that's what modern Psychology is trying to figure out.
[1] https://scholar.google.com/scholar?hl=en&as_sdt=0%2C34&q=cog...
They essentially say the brain can hold approx. 2.5 petabytes. I don't think this can be measured accurately (or that it makes sense to translate it to bytes) due to the nature of neural networks, but it indicates that it's huge.
Other studies say it's around 500.000 "thought objects" (whatever that means) that you can hold in your long-term memory. But outliers like Rainman and others suggest that this is not necessarily the limit.
Depends on the type of media being stored. The drive uses different compression algorithms for different types of data. We don't store video as video. We remember situations and relationships of objects, then draw those object's onto the situation during recall. This is a big deal in areas like eye witness testimony, or pilots flying a plane. Sometimes you remember that an object (ie an instrument) was at a particular location, recognize it, and remember it according to the previous memory rather than save a second copy. That certainly saves space on the drive, but risks loss of current details. An abnormal instrument reading may be lost when the pilot's brain substitutes an image from a previous memory.
The notorious Clyde, of Bonny and Clyde, broke out of jail with a bar of soap. He held it like a gun and told people it was a gun. Everyone remembered seeing him holding a gun. Their brains stored a generic gun image and afterwards had a very clear memory of the gun, which was only ever a bar of soap pained black with boot polish.
Apparently this scene was based on a real escape attempt by one Charles Makley which did indeed carve a gun out of soap and paint it black with shoe polish. He was however gunned down by prison guards, so the attempt was not successful.
He was the inspiration for the movie. But he wasn't very happy and felt trapped in his brain because no one was able to have meaningful conversations with him. Poor guy.
Reason for his insane memory is that the connection between the left and right hemisphere of his brain is missing. This means that all the information get saved because the filters aren't working properly.