A question about Zoryas

When you first see her in her snake form she is confused that she isn't under disguise. This seems weird to me because of the sheer height difference between the two forms of her.

How does her change work? Is it just a cloaking spell like the people stuck in bushes(So from her perspective she's always looking down at us, even as a human) or is it an actual form change from snake to humanoid(if so she should know what she is at the time.

There's probably a known explanation but this stuck out to me while I was doing that quest line so I thought I'd ask