If a grocery store makes 1% profit on each item it sells, and its debt service cost is 1% of its revenue (making it roughly "1% of its cost") a doubling of debt service cost wipes their margin to zero.
Say you make $100 in revenue and $1 in profit, and $1 of your $99 in costs is debt service. Your debt service increases to $2, your profit drops to zero.
Now your revenue increases to $101, presumably your debt costs stay fixed (this is not a guarantee - revenue expansion costs money), but your non-debt-service costs scale as well, and they are now 0.98 * $101 + $2 in debt service = $101.98. Congratulations, your profits are positive again, but they are $0.02.
I am eliding here the general difference between fixed costs and variable costs, and so it's probably not true that your costs would scale quite this much with revenue, but it's much closer to the truth than that you'd be back where you started, esp. in a low margin business.