The issue is not thrust in this instance, with the .300 Weatherby it is the case diameter at the shoulder. The Weatherby has its shoulder placed farther forward, into the portion of the barrel which is tapered.
.300 Weatherby.....0.492" @ 2.384"
.300 Winchester.....0.490" @ 2.196"
The Weatherby shoulder is almost 0.20" further forward, reducing the thickness of the barrel steel too much. The SAAMI presure limits are virtually the same for each cartridge, 64,000 psi for the Winchester and 65,000 psi for the Weatherby.