The real reason the number of partakers has gone up is found in the W07 5/1 qfr.
The number of partakers was around 8,000 for many yrs. It was believed that the calling was finished in 1935. That 07 WT did away with the 1935 cutoff date and stated that no one can say when a specific time or yr the calling ended. After 2007, the numbers of partakers shot up after that.
Since Armageddon didn't come, they had to justify the new members of the GB. All of them were baptized after 1935 and had to do away with the cutoff yr.
"On the other hand, as time has gone by, some Christians baptized after 1935 have had witness borne to them that they have the heavenly hope. (Romans 8:16, 17) Thus, it appears that we cannot set a specific date for when the calling of Christians to the heavenly hope ends." - Watchtower 2007 May 1 (QFR)