Take this field armour of King Henry the VIIIth or armour from this time period (~1500s). How much give or excess room would there be for a person who was taller/shorter or fatter/slimmer that wore that or similar armour?
Subsequently, the armor was given to William Herbert (ca. 1507–1570), first earl of Pembroke, Henry’s esquire and an executor of his will. metmuseum.org
The article says it was inherited by William Herbert (who on cursory glance was not as large a man), if he ever wore it himself what measures would need to be taken to make it 'fit' properly? Likewise if someone who was larger (in height or weight) was intended to wear such an armour (say somebody with a 72in waist instead of Henry's 58-60in waist, or a foot taller).
Fabric can be made bigger or small to fit a new wearer in numerous ways, but you can't use a thread unpicker on metal be it rivets or welds as far as I'm aware. Removing fabric requires scissors, or even just a fold and some extra sewing to make a better fit. Not so with metal armour, and even if you did the armour needs to be protective as well as fitting.
In essence: was armour from this period ever taken in or let out like clothes or otherwise modified?