[Gormo]

American here. I've never once in my 40 years on this earth heard someone say "six pound five". That parlance is often used for feet and inches (almost always in reference to human height), but I've never encountered anyone using it for pounds and ounces.

[nkurz]

I'm American, have lived in many parts of the US, and I agree: "six pound five" is not something that we say. As Gormo says, this construction is frequent for feet and inches ("six foot five"), but would not be standard for weight.

And personally, if I were to use this nonstandard construction, I'd probably pluralize "pounds". I don't know why "foot" is usually singular in the parallel construction.

The most charitable interpretation I can come up with is that your relatives are from the American South, and are actually saying "six point five", meaning 6 and half pounds, and you are mishearing them.