100% (or 1:1) crop means that each pixel of your image will be displayed as one pixel of a display device. For instance, this forum sets a 1200-pixel limit on the long edge of the photo. 1:1 cropped photos are often used to show the original sharpness of the capture. If you want to post a photo that displays on this forum without further downsampling, you can crop it in LR so that the long edge is less than 1200 pixels. Then you can export it without any reduction in resolution.