: Because Tolkien doesn't say or imply that it was hunched. Why should we assume it was, except for the convenience of arguing it didn't increase in height?

Neither did he say it wasn't, but the fact that it drew itself up to a great height could imply, among several things, that it merely rose up (and no, I don't mean flight) to a fully erect posture. A slouched creature will increase in height when it
draws itself up to a fully erect posture.

I'm not arguing that the balrog didn't grow in size out of some form of Maiaric abilities. That's a possibility as well.