Yamahas’s

I think it’s because they have a dry sump design. Less oil, less fluid drag.

With dry sumps, you only 'need' enough oil for the tank to be up to level (often 2/3, with the remaining 1/3 being air, not saying this is accurate for the Yam).

Volume is likely dictated by size/packaging constraints on engines as compact as these.

Because they only hold about 50% less…..

Because’s that’s’s the’s way’s the’s engineers’s designed’s them’s 

The 250f has been a wet sump since '14.

The 450f went to a dry sump in '23, but the oil capacity was unchanged from the prior year's wet sump.

because oil only makes you slip the less the better

((((  Paging @Falcon for analysis in regards to thread title  ))))

Haha.

Well, here it is, folks, my favorite pet peeve.
You could say "Yamahas's" in one specific context: if there was something owned by or belonging to multiple Yamahas. For instance, "the Yamahas's oil sumps are all smaller than those of other brands." This would mean ALL Yamahas, or at least a predetermined group of Yamahas, such as YZ450s and YZ250s. (You could write either "Yamahas's" or "Yamahas' ," without the last s.)

However, we often refer to such groups as "it" instead of "they." For instance, "The YZ250F has a small oil sump." Since we know that each example of a model is identical, we know that the speaker is referring to all individuals within that group. This is more likely how it should be phrased.

Furthermore, we often refer to the motorcycle model as a result of the corporation's work or specification decisions, and thus the property of that corporation. (Yamaha's design. KTM's hydraulic clutch. Triumph's new model.)

So, in conclusion, I think the OP could have written "Yamaha" if he meant to point out a difference between that brand and others, "Yamahas" if he were referring to multiple units or models, and/or "Yamaha's" if he were referring to the oil sump design or another feature belonging to the brand. "Yamahas's," while correct in a very specific context, is probably not what he meant.

To me, the volume of oil is solely what the manufacturer stipulates. They design the motor with clearances and volumes for the life span they are targeting. Wet or Dry sump is more about where the oil is housed. Your basic Chevy V8 is a wet sump motor. The pump is submerged in oil and provides it to the oiling points as designed. As OldNBroken stated earlier. racers are looking for any possible advantage. The Reciprocating assembly (Crank/Rod/Piston) running in a constant bath of oil causes some fluid drag. Most of your race cars (F1/IRL/NASCAR) all house it in a separate tank to reduce this fluid drag. Strangely or maybe not, I have several of the dinosaur type bikes (Rickman/Cheney/Other British bikes). They use a strategy called Oil In the Frame (OIF). Using the frame as a storage container. A lot of the earliest bikes used an oil tank or "bag" as storage for the oil. To give you an idea, my Rickman/Matchless sport svelte 3.5 quarts of oil! Of course after the first moto, half a quart is on the bike, half a quart is on the track and half a quart is on me!

Yamaha’s what?

Oil required in reality is a range. My kx450 requires 1qt but if I put in .80 qt or 1.25 qt it would be fine. People overthink maintenance me thinks. Change the oil frequently with enough oil to keep motor cool and change air filters. 99.9% of bikes will be great for years. 

So, to summarize - are you saying that the way it was written by the OP is the possessive of a plural?

Now! we know why Tomacs bike was smoking at Daytona.