If the text is smaller than the comp size you will get a different corner pin size. After Effects applies corner pins assuming the composition dimensions rather than the layer dimensions
To get around this:
- Place the text where you want it in the shot
- Pre-comp your text layer (moving all attributes into the comp).
- In Mocha, you will need to then use "Align Surface" on the same frame that you precomped the layer in AE. This will blow up the surface to the full dimensions of the frame and make the tracking data to be relative to this change.