I don't agree with this. The investors have to be just as sure of the risk involved in each valuation as before in order to have the same expected value for the overall portfolio. However, investing $400K in 5 companies instead of $2M in a single company will reduce the variance of the return on investment.
I think it's a tradeoff for the VCs between variance in the portfolio and the amount of work involved in finding 5 times as many companies. Given the amount of funding they deal with, I can understand them leaning towards the companies looking for $2M rounds.
It seems obvious. But I've proposed to several VC firms that they set aside some money and designate one partner to make more, smaller bets, and they react as if I'd proposed the partners all get nose rings.
As I pointed out above, the partner has to be just as sure of each of the 5 bets as he would be of one. I'd react the same way if someone suggested I'd do better at my job if I worked 5 times as hard.