In your PowerApp app, when you bind an Image control to a Picture property from a Person Field in SharePoint, by default it will display the low resolution small sized profile photo.
If you explore the URL string that is returned by the Picture property, you will clearly see why:
It requests the Small size version of the photo.
While you can’t change the value returned back from SharePoint, you can easily replace the string Size=S with Size=L so the Image control can load the large image instead.
Select the Image control and then use the following for the Image property:
Replace(ThisItem.Candidate.Picture, Find("Size=S", ThisItem.Candidate.Picture), 6, "Size=L")
This will now load the large size!
p.s. Yes, I have already reported this bug so we will work on fixing this soon!