Fair question. An open workbook (itself) takes up a relatively small amount of space inside of Excel. Probably less than 300 bytes counting a long file-name string, file path, and file properties. However, workbook content will include one or more worksheets, which contain some amount of data. When you open the Workbook, Excel will try to load all of it and there is where your memory will be consumed. If it is actually a "virgin" workbook, then you would be quite right that its load would be minor. But if it is a full workbook, ... not so minor.
The part about system resources is that Windows has a limit on the number of files it can have open at one time and you would not believe how many files Windows itself will have. Open the Windows Task Manager and look through the various Processes and Services. EVERY ONE of the entries in those panels of Task Manager represents at least one open file.
Windows reads the .EXE file to load its content before execution. So from the viewpoint of the file system, an image file is just another open file. I would have to get into Windows paging dynamics to tell you EXACTLY how it is used, but trust me when I say that it counts as an open file that STAYS open for the duration of the process that opened it.
Where that open file gets significant is that this is not a part of Access that is affected by the open file. It is a part of Windows, and unfortunately, Windows will automatically allocate a fixed number of file slots (actually called "file handles"). If you open too many user-level files at once, you run out of a system resource - open slots in the open file table. You can fix it by finding the registry entry, but the pre-allocated file table does not change until next reboot, because it is a boot-time-only parameter.
As to Data Modeling: Other than saying that there is something on one of the ribbons that has something o do with data modeling, I really can't tell you much. I do my modeling in Access, which you might have guessed. However, for whatever reason, the Excel "limits and specifications" makes this distinction about modeling, which means they think it makes a difference. I believe it has to do with what is pre-loaded when you launch a spreadsheet with data modeling involvement. Beyond that, I'm out of my depth.