I don't understand why the comparison even makes any sense:

Let's say debt is equivalent to the mortgage on your house. You compare an individual exposure by their monthly repayment against their income, not income against mortgage amount! Why is it any different for this?

Usually in France, it's customary to allow this ratio to be 30%. With median income at 20k€, this leaves about 5k€/y, or 410€/month. At current rates, mortgage size of 100k€, total cost of 150k€. The "debt to GDP" is thus 150/20=750%.

Apples to oranges?