I came to know about Zeplin through one of our clients. Zeplin is a collaboration tool between designers and developers. It helps in generating style guides and resources automatically.
I faced an issue when exporting the assets from Zeplin. The images size (@1x, @2x, @3x) seemed to differ than the required size. I found that, it is because I had chosen the wrong screen size (density) while importing the Sketch file (screen).
So when you import the Sketch file into Zeplin, make sure you choose the right density same as what you had used in Sketch.
I wanted to share this, since I could not find any official explanation and hoping it would be of some help to people facing this issue.
To know more read Zeplin – Collaboration between UI Designers and Front-end Developers