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First, you need to hook up scaling to your instances.
In the Attribute Editor for your particleShape node, under the Instancer (Geometry Replacement) section, set the “Allow All Data Types” box.
In the General Options subsection, change the “Scale” drop-down to be radiusPP. This will scale each instance by the radiusPP for each particle.
Second, you need to add a runtime expression to change the particles’ radiusPP over time.
Lower in the particleShape attribute editor section, in the per-particle (array) attributes section, right-click on radiusPP, and choose one of the run-time expression options.
In the expression editor, create an expression that will shrink your particle’s radiusPP as it ages. I set my particle’s lifespan to be a constant value of 3, so I used an expression like this:
particleShape1.radiusPP = 3-particleShape1.age;
That ought to do the trick.
Take note: without setting the particles to die at age=3, this will make the radiusPP go into negative values, so your instances will become inverted and larger. In that case you’ll want to clamp the result of this expression to avoid negative numbers.
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