Behold! an absolutely terrible mixed metaphor, demonstrating precisely why you shouldn't do that. I cannot fathom what is meant to be represented by the surface area of a fat, unflexible, heavy, poorly rolling, defective "asset." It also seems to conflate weight and surface area, which aren't even proportional.
Right from the start, I would suggest you abandon all hope of ever understanding this word soup of a sentence. In technical jargon, the "surface area" of a product is the part of the product that is exposed to the outside world, and therefore could be vulnerable to an attack.
Technical debt, meanwhile, is the concept that as a product gets more features, components that made sense initially can become overloaded or too complex to maintain. It is a natural part of product evolution, and is eliminated by redesigning parts of the product to work more effectively and robustly.
Functional debt, while not a term that I'm familiar with, likely represents the concept that there are more desirable features to add to the product than there is time to add them, or perhaps that customer demands outpace the means of the producer to meet them. This is not a normal part of product evolution and would be indicative of needing to invest more resources in the product.
I have never heard of "surface area of technical debt," and the concept doesn't make sense because technical debt is not something introduced into the product from the outside world. Technical debt is more reasonably compared to a volume than to a surface area.
"Surface area of functional debt" is similarly nonsensical, since functional debt (I assume) is characterized by features not part of the product. Thus, the design and implementation of the product is wholly irrelevant to the accrual (or not) of so-called "functional debt."
I am very suspicious that perhaps the author of this paragraph really has nothing meaningful to say and is attempting to hide that fact behind syllogisms and jargon, and if they're trying to sell you something, run away.
TL;DR: The only person on Earth who knows what this phrase is supposed to mean is its author, and if they didn't explain it, nobody else can.