I have had/am having the same problem. I have run all Microsoft updates and Office 365 is automatically updated so there is nothing to manually update but the problem persists. I have resigned myself to trial and error to get the sizes of buttons consistent.

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Reply from Richard Rost:

OK, I spent about half an hour Googling this problem. And one guy said that he figured out the problem was because of a display setting in WINDOWS.

Go to your WINDOWS Control Panel > Display > Change the size of all items.

Make sure that this is set to SMALLER (100%) which is the default setting. If it's set to either of the larger settings, this problem occurs.

I am unable to get the problem to occur on any of my systems, so I can't verify the solution, so if you try this and it works PLEASE let me know and I'll post this as a bulletin in my Blog because I know a lot of people have had this problem. It must only be with certain display adapters.