Hi Bill,
Yes, I see what you mean. The lightbox was not created with screens that resize in mind (before the mobile era) but it does adapt to smaller screen sizes fairly well: after resizing your browser window, close the lightbox then click the link again to open it for a second time. You’ll see it’s now fitting inside the smaller window.
In most cases, visitors will not resize their browser window while browsing very often. Or at least won’t be very surprised if not everything scales along perfectly. Only on mobile devices where a reader opens the lightbox in portrait position and decides it would be easier to read in landscape, he would tilt the device to landscape position and find that the lightbox does not get wider than before…