Just came across this phrase and upon breaking it down it looked like it would have the opposite meaning to what it really has (高を括る-to make light of). Have I missed something and there's actually a good explanation for this phrase? My first thought was that it would mean 'to expect a lot' or something similar.
高【たか】 means "yield/amount/quantity" (e.g., of a crop). くくる means "to bind/tie" and, by extension, "to summarize/consolidate". Thus, the literal meaning of 高をくくる is nothing more than something like "to estimate (the capability of someone/something)" or "to assume". However, as an idiom, it has gained a derived implication of "to (wrongly) estimate/assume", and that's why "to make light of something" is a valid translation. When it comes to idioms, you cannot expect their literal meanings are preserved.