With the kind of crowds Bolsonaro attracts compared to Luis I find this highly suspect.
A politician who attracts a more visceral audience is going to superficially look more popular than they actually are.
So going by crowd size can be deceptive.
One explanation is that voters may be enthusiastically voting "against" a particular candidate, in which case they would likely not be attending any rally or showing support in any meaningful way.
I'm not sure if this is the strongest argument in favor of your hypothesis. In most instances it would rather be an indicator of election fraud.