A couple of reasons why the rent supplement bill would be larger than the mortgage interest supplement bill:
1. If you have a place on a local authority housing list but the council cannot accommodate you then you qualify for rent supplement if your current housing is inadequate.
2. Rent supplement can be a long term payment. Some recipients would be claiming rent supplement for over 6 years. So there hasn't been a steady increase in claimants, rather there are people who were claiming before the recession who are bumping up the figures.
3. Mortgage Interest Supplement is viewed as being a short term handout. I know of 2 families who have had their supplement removed in the past year. The supplement is granted for an initial 12 months (or less) and is not automatically renewed. If the claimants don't make efforts to reduce their mortgage responsibility (e.g. take in a tenant/come to reduced payment arrangement with their lender) then they could lose the payment.