I think there are a few things to bear in mind with this comparison.
How many actually congregations do Pentecostals have? I doubt there are as many times the number of congregations of Pentecostals as there are adherents when comparing them with JWs.
The number of JW congregations worldwide is actually pretty impressive. Only the Catholic Church and a couple of other church organisations have more congregations worldwide than JWs.
And you are treating all Pentecostals as one group. Pentecostals consist of a huge number of different organisations with variations in belief and practice.
Plus Pentecostals are considered by most to be a denomination of Christianity, with mostly orthodox beliefs, so conversion to Pentecostalism is not as great a leap as it is for those leaving Christian groups to join JWs.
A huge part of Pentecostal growth has been in Latin America where millions of Catholics moved over as part of a wider political and social phenomenon that involved breaking up traditional power structures including the Catholic Church. There has been a trend for some who joined the Pentecostal movement to go back to the Catholic Church as the church attempted to address the concerns of many who had left.
My own observation of Pentecostal churches in the UK is that the long-standing groups such as Assemblies of God and Elim Pentecostal are subject to decline as all mainline churches. The only Pentecostal churches continuing to enjoy any growth are the ones supported by immigrants from Africa and the Caribbean. In my town the Assemblies of God Church closed a few years ago and the Elim church has declined significantly. There is a local variant of Pentecostalism which is peculiar to the area, and resulted from 1950s and 1990s (Toronto blessing) revivals. But this small group has declined over the last 20 years from around ten churches locally, to three small churches.
Pentecostal churches generally are characteristed by temporary bursts of activity and expansion, followed by decline and closure.