WhatsApp => Shift+Enter problem

Hello,

I'm new in the apple community and I have my Macbook Pro since one day. I downloaded WhatsApp as App from the the App Store and started typing without any problems, but the problem is, when I wanna break the line for a whitespace between two paragraphs or only for starting a new sentence, then I get one or two more whitespaces more and I just wonder why. When I use WhatsApp in Safari then the same happens, but(!) when I copy a line break into the app and paste it there, then it works.

Why is that like this?

Is it because of any other shortcut of OS X?

Many thanks for help!

Best regards **