TCanvas retains old ClipRect on resize

I've got a TImage component aligned to alClient, so it pretty much
takes up the entire Formspace (except for the TPanel, which is at the
bottom and contains various buttons, etc.)  I'm generating the
graphics (a simple graph, in this case) manually using the Canvas
component's drawing methods.  When I maximize the window, the TImage
does grow accordingly, but the visible area is limited to the old
size.  Even the grid lines are scaled to match the new size, but all I
see is a small white window of it -- the rest is grey.  A check of
Canvas.ClipRect reveals that the clipping rectangle has not changed to
the new size.  What am I missing here?