I gave you a reason why they look larger and a second reason why they wouldn't be custom. Deal with it.
Making "custom" turn signals that are maybe a few mm larger seems stupid to me, when the stock ones work and have no fitment issues. If you are going to make turn signals, why would you make them almost the exact same as the stock ones, but have to modify everything else to get them to fit? You can go a totally different design. The amount of effort in going something different is almost the same effort of doing as you imply.
When you open the door of fabbing your own parts, designing around stock parts is merely a guideline.