Why would copy trading work?

I realized there is quite a few vendors out there which allows retail traders to copy the trade of other "profitable" accounts which they have.

Why would this work? The size of the accounts would be different, how can you be sure you would be able to follow through the trades even if those accounts are truly profitable.

It's very easy.

All the accounts are based on account size. Say the master account that is being traded is $10k. Each sub account or slave account that the master account is linked to will then reflect or allocated lot size based assets in the account. Say you have 2.5k account which is 25% of the master account therefore if a 1 lot was placed on the master account you will receive a 0.25 lot.

It's very easy.