Propagation delay doesnt depend upon the size of the packet.It depends upon the speed of the signal in the channel.
Rather, I think its a channel property. Its like the difference between 8 lane highways and 2 lane highways.
I say to Lambhorghini that drive on 2 lane highway. It will go like Maruti Alto. But if its 8 lane road, it will be like superfast bullet train.
Here the car was same, channels were different.
Similarly, packet is always the same but the time required to cross a channel depends upon the channel I think.