It sounds like a reasonable result I think. There’s nothing in the context or in the content passed to measure with a list, so nothing about a list is going to affect the result, only text settings.
You could look at the function debug-font if you want to know more about the size of text. If you want to use the current text size (here 12pt) as a length, it’s already available as 1em.
I should add for clarity: every letter will measure as the same size if we don’t change the method. We can measure the actual bounds of the letter like this:
#context {
// Measure the height of capital "T" with current text settings
let t-height = measure({
set text(top-edge: "bounds", bottom-edge: "bounds")
[T]
}).height
t-height
}