Questions not answered:
You lend 100 euro with a salary of 100 euros and 500 euros to someone with a salary of 1000 euros.
The first owes you 100% of salary while the second owes you 50% of salary.
By what logic would we hunted the first that fails to pay its debt and leave the second they can afford? Why try to get 100 and not 500?
The main part of your question boils down to the perceived ability for a country to pay back its debt overtime. If the IMF fears that the lack of economic growth will cause default on the debt then they are going to be more proactive in collecting and preventing that default by whatever measures they deem necessary.
It may not seem fair from your perspective but the debt GDP ratio is only one statistic out of many that are involved in the evaluation of an economy. Often is it advantageous for a country to take on debt that they feel with help propel economic growth. But if a country over burdens itself with debt then the opposite can happen. And the IMF beleives that this is the case in Greece.