because India's growth has a direct consequence on your security paradigm
The more India faster grows the more money it has to spend on defence, It's also inflates India's regional as well as International clout and In consequence of it pakistan feel itself more vulnerable and insecure.
So from the pakistani perspective they should grow at least what India has been growing otherwise the gap between two countries is just going to get bigger by each passing year.
look at the chart below to understand this
and then watch this video
so it's important from pakistan's perspective to maintain parity between two countries