Except the amount doesn't matter, its how the investment is made that matters. That's investment 101.
Look at Afghanistan, and Iraq. It's estimated that the US has invested more than half a trillion dollars in infrastructure, security, education, and energy projects, but 90% have been completely and utterly worthless. The bigger a project gets, the easier it is for it to fail.
The reason why businesses are moving to India is because of the long term impact. A dollar put into buying Indian stocks may grow double or triple within a few years.
Also, don't just look at total amount, look at percentage wise, how much that's worth. What may be a large investment in Somalia, would be a simple write off for most other nations.
An investment of $75 billion is an okay amount, if you take into account India's huge economy, and it's potential impact on India's economy. While, Pakistan being a middle economy, $46 billion is a large investment (almost a fifth of Pakistan's nominal GDP), which takes into account that the Gwadar port is set to become one of the busiest ports in the region, and one of the largest in the world, thus the long term impact (if the project succeeds) is huge, with potentially massive return on investment. That's not even mentioning the money would bring in much needed jobs, work on energy projects (doubling Pakistan's current capacity) and/or upgrading and building new (much needed) infrastructure.
$46 billion is chump change for India, but taking percentage and rates into account, the equivalent amount for India (if India's current GDP is $2 trillion) would be around $400 billion, meaning it would take this much to have the same type of impact on India's economy.