Holy cow, I wrote a book!
why the DrawIcon function draws at the default icon size
instead of respecting the actual icon size.
After all, if you loaded a nonstandard-sized icon via
LoadImage, then presumably you want to use
that nonstandard size.
The question is one of those types of questions that fails to
like asking why NASA didn't send the space shuttle to rescue
the Apollo 13 astronauts.
At the time the DrawIcon function was written,
the LoadImage function didn't exist,
and wouldn't exist for over a decade.
LoadImage function showed up in Windows 95,
but Windows was drawing icons long before then,
and for a long time,
the only way to load icons was with the LoadIcon function,
which always loaded icons at their default size.
When the ability to create nonstandard-sized icons was added,
you then had the question of how to draw them.
Code which relied on the fact that all icons were the same size
would call DrawIcon expecting the result
to be a 32×32 image (or whatever your icon size was).
If you drew it at its actual size, you would either have
this L-shaped "hole" in the application (if the actual size was smaller),
or you would have an icon that overflowed some other part of the application.
Either way you lose.
Therefore, DrawIcon always draws at the standard
Think of it as DrawIconBackCompat.
If you are a fancy new application that can handle icons at nonstandard
sizes, then use DrawIconEx and don't
pass the DI_DEFAULTSIZE flag.
The documentation states that the DI_COMPAT
has no effect.
Presumably it had an effect in some previous version of Windows?
In Windows 95, if you used the
LoadCursor to load a standard cursor
(like, say, IDC_ARROW),
but the standard arrow cursor was customized by the user,
Windows would draw the customized cursor.
Passing the DI_COMPAT flag forced the standard
arrow cursor to be drawn.
So far as I can tell, nobody ever passed that flag.
My claim that nobody passed that flag is incorrect.
The DrawIcon function itself passed that flag
(and still does today, even though it no longer does anything).