Adding to the outside of the letter throws up problems because of maintaining consistent vertical dimensions (x-height, Cap height, ascender height etc) between weight variants in the same point size. For example you wouldn’t want a bold word emphasised in a line of roman to jump in x-height.

Having said that, weight can be added to the outside of the letter but mainly in a horizontal direction e.g. stem thickness, which is why bold type (generally) sets a longer line than the roman.

That’s the simplistic answer, I’m sure someone can expand on this in a more eloquent fashion.

There is no definitive answer in creating bold weights. When I do it, I add weight to both inside and out but it is not that simple. The bolder the weight becomes, the more vital the shape of the counter becomes. Counters in bold weights are surrounded by enough black area that they become more prominent as shape, The figure-ground relationship is a delicate balance and of profound importance in type design. Simply adding weight is easy. Making the form and counterforms balance and serve each other, is the hard part.
Letterspacing and glyph-to-glyph fit are also of prime concern. The negative shapes between letters must work with the counters and the forms. It is like juggling, you have to keep several balls in the air while you catch one of them then throw another.

ChrisL

Mostly because that way you can get bolder faster. And since darker fonts need (in fact require) less interletter space, less internal space is better too. Another reason is to avoid making the bold too wide.

BTW, I have encountered one quantification of how much wider a bold should be compared to the roman: ~8%. This from that old Letraset font editing program - what was it called? I can’t believe how shaky my memory has been getting lately...

hhp

FontStudio?

Yup - thanks.

hhp