The trend in the BoC's asset-to-GDP ratio is quite revealing, because it demonstrates that the bank's operations, over time, are functioning at a deficit relative to the local economy. This is a large problem now facing the European Central Bank, the Bank of Japan, and the Federal Reserve - all of whom seem to be having trouble with reducing the size of their books relative to the local economy.
Right now it may not be a problem for the BoC, but it can quickly escalate. For example, Canada's banking sector has created a massive stockpile of credit for financial transactions. This includes CAD $1.18 trillion in mortgage credit. If only a tiny fraction of these mortgages become non-performing, then the BoC will need to purchase them through a "quantitative easing" program.
This is becoming a threat to the BoC the more that house prices fall, as is currently the case. And it becomes even more of a risk if economic growth begins to falter. Right now, it's uncomfortably close. The assets supporting mortgages (houses) are marked-to-market, so a contraction in housing prices may seriously increase the risk of these loans. If the economy contracts and people lose their abilities to maintain their mortgage payments, some of the mortgages will turn into non-performing loans.
The BoC would be quickly forced into a situation where it would need to purchase a portion of mortgages held by banks and park them on its balance sheet under a "quantitative easing" program. Otherwise, the banks may refuse to lend any money. For example, if only a generous 1 percent of the mortgage market became non-performing, the BoC would need to buy them up, and that would add about CAD $12 billion onto its books.
If that generous scenario were to happen today, we would see the BoC's balance sheet increase by about 10 percent. To make matters worse, if people are not making payments on their mortgages, it's very likely that the rest of the economy is not doing well, either. Assuming a generous economic growth rate of zero, we could see the BoC's asset-to-GDP ratio increase significantly.