Total debt is not only how much to service credit debt, but total expenditures also.
In the case of individuals, they have their credit debt and then there is such things as insurance, food, clothing and if their home is not part of their credit debt, rent. Put all of that together and that is their total debt. An individuals "GDP" is their net income, or gross income if you want to add taxes to the total debt. An individual can sustain 300+% "GDP" because they do not apply all of their "GDP" to total debt, if they do, then they go bankrupt. An individuals GDP is usually equal to their revenue. I have a house and a car, my only credit debt, approximately 217% of my revenue. I pay $1,500 or approximately 30% of my income to service that debt, leaving me 60% of my revenue for other debt, since my total debt is far below my revenue, less than 50%, I can easily sustain this.
In the case of a country, first they do not get 100% of the GDP as income. They only get a percentage of it. Their revenue is not equal to GDP. Next, 100% of their revenue is used to service total debt. For example, the US has tax revenue of 26.9% of GDP, that means that our credit debt at 100% of GDP is actually around 400% of revenue. In the case of Japan, their revenue is 28.3% of GDP, so their credit debt of 229% GDP is actually approximately 700% of their revenue (very rough estimate, if you want to do the math, go ahead). 100% of a countries revenue, even without credit debt, is still spent on debt. A country, with 0 credit debt still pays a total debt of 100% of revenue, if not the tax payers are going to scream about paying too much taxes. So a country like the US, which has about 400% credit debt, we pay 5% of our revenue to service credit debt, but the rest goes to other debt, so the total debt comes in around 595% of revenue. With 100% of revenue already going to debt.
That is why the first is not really a problem while the second is a massive problem.