Entry Custom Renderer Frame.Width is always 320

I'm writing a custom Entry class to have Entry controls on iOS only have an underline. In my custom renderer, the Frame.Width is always 320 no matter how wide I make the control. I draw the underline that size and the entry field is obviously longer because the typing goes beyond the end of the underline.

I'm doing the drawing in OnElementChanged. Should I be doing it somewhere else?

What am I doing wrong?