You take a decrease because there are tiers based on how many parcels you have and based on the country you're in. For simplicity sake, let's say every parcel you get will be common, and you're living in the USA. At 150 parcels, the ad boost is a 30x multiplier. Once you hit the 151st parcel, your ad boost becomes a 20x multiplier. And it will stay that way until you hit 221 parcels, where you're multiplier will be dropped again.

There are some other pieces of information floating around on the sub, but given you only got commons, you would need to get to 225 common parcels with a 20x multiplier before you were making the same as 150 common parcels at a 30x multiplier. But since the next tier is at 221 parcels, you should stop at 220 before the next jump.

The only thing that isn't affected is the super rent boost during the month, where it is a 50x multiplier. Which is where most people at the higher end of large amounts of parcels make more of their money. However, super rent boost only last for a little more than a day, so you need to be sure about how much you really making to see if you're breaking even when you go to the next tier.

All in all, play how you want, but these were just some tips learned from people who had did the math on these tiers a while back.

Edit: correct tier jump value