The doctrines taught and methods embraced by false prophets result in resistance to the truth. False prophets are covetous of followers. Hence, false prophets are very likely to boast in the number of people that attend their churches. Please note that this statement is not meant to condemn everyone that mentions the number of people attending a service or church. Boasting in numbers and simply mentioning numbers are two different things.
Boasting is related to pride.
The number of people attending a church is deceitful if the ratio between attendees and sheep is extremely different. In other words, how many people attend a church means absolutely nothing if there are only a few sheep. When the truth is preached, evil people leave. This also implies that the false leaders are aware of the lies when they tell them. There are many spiritual imposters in the world spreading their wares.