The file fetched from the internet is 40MB not 50MB. It opens fine and re-opens fine as long as the built-in data query is not run.
Where I use Excel, data connections are not automatically enabled and refreshed by default. I suspect this isn't your case. This can be configured by going to Excel options / Trust center / External content / Prompt user about data connections.
So that could explain the problem. Disable data connections so you can verify if the file loads entirely. And only then, as you are prompt for enabling data connections, can you enable and then refresh it.
Unfortunately, running this query implies that the database being targeted is accessible which is not my case (OLEDB datasource).
I suspect after each refresh the data could grow up significatively, and that could be seen after hitting save. Especially if you keep hitting refresh over and over again.
Additional suggestion : in theory, Excel does not save external linked values which is one way to explain why the file grows, but you can try to avoid that by going to Excel options / Advanced / "When calculating this workbook" and uncheck "Save external link
values".