I think it might be a graphic card issue? Because it works fine on my laptop and not on my PC...
The capx file you have is exactly what I have, the grow rate should infinitely expand a particle until the time you set out as a timeout. Even if I put 1000px as a grow rate and 20 seconds as TimeOut it still stops at a certain size. Not covering the entire screen.
We tried on a bunch of different computers, and it's working for the others, not mine. So Construct people, if you read this I think we might have a real bug here
this is my graphic card : Geforce GTX 780 / PCIe / SSE2 /