No, Germany could not issue such a fine for a minor single violation. "Proportionate" is a so-called indeterminate legal term that is open to full judicial review with no interpretative power left to the administration. Our law is littered with clauses stating "fines up to 50 Mio. €" and courts have developed a system of how to make sure everyone gets treated fairly. Other European countries have the same. This is simply a difference in legal traditions between Europe and the US. We don't simply cling onto the exact text of a law and call it a day. Interpretation of laws goes far beyond that. It may be scary to someone not used to such clauses but I can assure you companies around here do business just fine even though they have to comply with a long list of such laws threatening fines this high.

Anyway, "proportionate" isn't as ill-defined as you claim. It's a term of art used in many laws and we (and the courts) know pretty well what the legislator meant with it. Heck, they even list some criteria. If those aren't fulfilled a maximum fine isn't possible. Also, "proportionate" implies that a more severe violation must have a higher fine meaning the maximum can hardly ever be reached.

A law isn't code and shouldn't be read as such.