That sounds like something is slipping on the floor perhaps? That is far more error than I could explain.
BUT if you really wish to compensate in a linear way you can play with the wheel radius we default to for our wheels. We have a .yaml file that holds this radius but keep in mind if you do an apt upgrade this file is overwritten so take a note of what you did.
Look at /opt/ros/kinetic/share/magni_bringup/param/base.yaml and look for wheel_radius.
Again keep in mind that ANY slip will throw off the best of odometry.