It's a strange one you would think those with the highest debt in relation to GDP would be badly run countries but the list shows some countries with low debt are proper basket cases and vice versa. Japan's is stupidly high for instance but plenty of African countries have low debt, maybe because no one will loan them anything.
That is exactly true, every country has a credit rating and higher risk lending is typically done at higher interest rates. Poorer countries also have typically poorer currencies so coupled with high interest rates it is extremely expensive for them to borrow against a foreign currency. More often than not instead of borrowing they just print more money. It is different when you have foreign currency on hand though which is why in many poor countries the Dollar is so widely accepted.
The biggest countries are most indebted because they can often afford it and have more capacity to control it. They also have access to reserves or funds at favourable rates when things do go wrong like with the IMF and others who pool resources. Poorer countries don't have any of this and I bet a portion of any money they do borrow or otherwise is syphoned off into someones Swiss account anyway..