This could be because you are using Dan's branch that is not shipping “DejaVuSansMono-Bold.ttf”
and trying to make bold the not-bold.ttf gives bad results.
I'm not using Dan's branch... I'm using the regular Pd Vanilla 0.47-1
trying it with bold doesn't change the objects size, as you said it didn't happen in windows...
It's just really hard on the eyes and makes it really hard to read any text because bold was never intended to be the "regular" default font... no font in the world was designed to be used as bold by default... or italic... or bold/italic... that's why they are "bold" or "italic" and not regular...
It makes sense to have those options just to highlight and outline some parts of the text, but pd's comment do not allow that... so I wonder why the possibility of hardcoding it to bold exists at all... let alone be the default, and if linux people really thought that was nice, I really wonder why and would like to hear about it - I just can't see why so far...