To elaborate: A window's content may have a maximum size because it's unable to handle larger window sizes. For example, it may not have correct autoresize masks set up.
I think you make a good point that resizing the window's content may lead to problems. With a bit of finagling it should be possible to keep the size of the contentView unchanged while resizing the window. Even though that's a bit of extra work, my guess is it will work better than trying to link two windows together. Given what I've seen in the past (e.g. with drawers), linking windows doesn't really work too well and is highly susceptible to platform-specific issues. Implementing sheets as views would avoid all of that.