Under Russell the 144,000 little flock were glorified resurrected christians who went to heaven along with non-glorified Great Crowd christians who also went to heaven.
Rutherford saw his membership figures climbing beyond 60,000 and threatening to exceed 144,000.
This would be embarrassing. Consequently, a doctrinal change or "new light" emerged making the Great Crowd an earthly group of Jehovah's Witnesses distinct in kind and nature from the heavenly anointed.
Your point about how membership numbers forced the new doctrine of the great crowd ...
The increasing numbers of Bible Students affiliated with the Watch Tower Society did not force the change in doctrine. The Watch Tower Society taught the following in the 1917 publication The Finished Mystery on page 103:
FM103
Was ten thousand times ten thousand, and
thousands of thousands
.— The number of theGreat Company will apparently exceed one
hundred millions. Num. 4:46-48 and Ex. 28:1
indicate but one priest to each 2,860 Levites,
which would make the number of the Great
Company approximate 411,840,000.— T118,
119; Dan. 7:10. 5:12