This common belief has led many bochurim collecting money on Purim to dress as avreichim, to bring home the bigger pot.

The question was raised if this "deception" is allowed or not. On Purim can one dress up to appear as an avreich to collect tzedaka knowing that people thinking he is really an avreich will donate more? is this some sort of deception and theft?

Hamechadesh brings a psak on this from many gedolim including Rav Yitzchak Zilbershtein, Rav Shammai Gross, Rav Moshe Klein, and others, and in the name of Rav Chaim Kanievsky, saying that this is allowed. They explain that this is the custom and is acceptable and everyone knows that people are dressing up and they give whether it is a bochur or an avreich. There is no concern of theft by doing this. The qualification of this is that there can be no actual lying - the person collecting cannot say he is an avreich if he is not - he can only dress as an avreich. 

Not only that but Rav Gross added that you can go to the same house multuiple times knowing the donor is generous and gives openly. You cannot lie - if the baalebayis asks if you have already been there, you cannot lie or be a smart aleck - you have to tell the truth, but if he gives without asking and does not notice that he already gave you, that is fine.

The initial issue makes sense to me. It is Purim and people know that the collectors are dressed up and that they don't really know who is behind the costume. The second issue seems strange to me - people might have a dozen or a couple dozen collectors and likely wont recognize them all. If he is giving small amounts, he might not mind, but if he is giving significant donations, he might not want the same person coming a few times to take knowing that he won't be recognized. But that is Rav Gross's psak.