Maybe SetExplicitMinSize also needs to be used. Maybe the weight of the columns can be tweaked too. It’s hard to say without seeing the complete layout, as there are constraints coming from several places, for example, is the box itself allowed to be smaller, or is it constrained to remain this wide?
I was worried about something else in the layout being the issue initially, but other controls besides BButton worked with with just the BGridLayout cell sizing.