Mutual fund: SPAXX (trailing 1-year yield of 4.97%. ) is already a lot behind VMFXX (trailing 1-year yield of 5.28%). Is this normal?

Recently I checked the yield of several mutual fund: SPAXX (trailing 1-year yield of 4.97%.) is already a lot behind VMFXX (trailing 1-year yield of 5.28%). But SPAXX still has total assets of $328.18B while VMFXX has $303.43B.

I guess the difference in yield is largely due to their difference in expense ratio/management fees (0.42%/0.25% vs 0.11%/0.10%).

Can anyone explain why SPAXX charges so much for the fund? What are advantages of SPAXX so that people are choosing it over VMFXX?

One possible reason I think, is Fidelity brokerage account might be more popular than Vanguard, but I am not sure why simply because of this people want to buy mutual fund from Fidelity than Vanguard.