Squash and Stretch Scale
Squash and Stretch Scale
category:

This bouncing squash and stretch expression should make your animation a bit more alive by adding a proportional scale to your shapes or images

use:

scale

maxDev = 13; // max deviation in pixels
spd = 30; //speed of oscillation
decay = 1.0; //how fast it slows down

t = time - inPoint;
x = scale[0] + maxDev*Math.sin(spd*t)/Math.exp(decay*t);
y = scale[0]*scale[1]/x;
[x,y]