New to Acrobat DC, but I can't see what is going wrong here!
I have a file which is print-ready and therefore 54mb in size. When I edit this file and make ANY changes to it, and then save it, the file size reduces to under 1mb. When a colleague tries the same thing from his PC, the file size stays correct.
Any ideas what I might be doing wrong here, or is it a preference / setting I need to adjust?
Are you using a "File>Save As" and your colleague "File>Save"? That would explain why your file size decreases: Acrobat performers a different operation, and only saves what is actually necessary in the file, whereas a "Save" uses the original files and appends all changes to the end (so the file size will increase by the amount of modified/new data in the document).
If that's not the case, if you have Adobe Acrobat DC Pro, you can use the "Audit Size" tool to find out what is using how much data in the file and compare the two copies. You get to this tool by selecting File>Save As Other>Optimized PDF - then click on the "Audit space usage" button in the upper right corner of the dialog.