Global-only fields adding columns to content table? I understand the rationale of adding one column per custom field to the craft_content table in the database; but why would this happen for fields that are only used once globally?

Is there a way to define global fields without adding to the entire content table?

Perhaps a better option would be to define a single "Options" matrix field which can be used anywhere, and then use that to store zero-or-more options?
A field is a field, no matter where you are using it. It doesn't matter to Craft whether you attach your field to a Global Set or a Section (or Users or Assets)... All fields are treated equally.

Your fields can be shared between Element Types, and often are.