If you have not already seen it, MS is pushing out a fix to 365 to return the QAT buttons to 2013 size. If you don't already have it, just force a manual update of your 365. I don't know if the fix applies to standalone 2016 also.
As well, in some of the questions, people noted that switching from Mouse to Touch to Mouse mode does not always return the QAT buttons to original Mouse size. This next link has a potential simple fix:
Partial fix QAT buttons not resizing down fully- Change DPI Scaling
https://answers.microsoft.com/en-us/office/forum/office_2010-customize/huge-ribbon-and-icon-size-in-microsoft-office/32fabc50-d6a4-44f5-ae87-b1c332813d22
In addition to the link above to an unofficial site, MS also has an official page that lists work being done on Office 365:
Office 365 Roadmap - Fasttrack
http://fasttrack.office.com/roadmap
The Office 365 Roadmap lists updates that are currently planned for applicable subscribers. Updates are at various stages from being in development to rolling-out to customers to being generally available for applicable customers world-wide.
NOTE: there are 3 sets of filters available.
In the Black bar the the first set allows you to select the category:
Launched, Rolling out, In development, Cancelled, Previously released
Immediately below that there are check boxes for features Updated and/or Added within last 30 days
Finally, if you click on the “Filters” button, a section will expand to allow you to pick specific apps, services or platforms affected by the changes.