I don't know when this was changed, but in previous versions replacing the texture kept the size. The auto-resize can be very frustrating.
However, there is an easy fix: right-click on the texture path and select "Replace and retain size". It will keep the size of your existing texture, perfect for replacing.
Additionally, you can also use this to "up-res" textures when needed. For example, going from a 300dpi to a 2000dpi texture is easy using this method.