Because text extents are in user-space coordinates, they are mostly, but not entirely, independent of the current transformation matrix. If you call cairo_scale(cr, 2.0, 2.0), text will be drawn twice as big, but the reported text extents will not be doubled. They will change slightly due to hinting (so you can't assume that metrics are independent of the transformation matrix), but otherwise will remain unchanged.

Namespace: Cairo
Assembly: Mono.Cairo (in Mono.Cairo.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0