Yes,that's correct...a carb is designed for a certain engine situation. The Holley 600 ,out of the box,will be jetted just a touch rich ,for safety,at full throttle on typical 350 cube V8.Same for similar sized Carter and Edelbrock.
In theory the Holley 390 is pretty close out of the box for less powerful engines,like 180-250 hp.But not always.
The simple carburator size test;rig a vacuum to the intake,run the engine from let's say 2000 rpm to redline in second gear.When you first go wide open the vacuum gauge needle will zero then rise up a bit as the engine nears redline.If the vacuum rises to about 3-4 inches ,a larger carb may help.
My observations found that GMC 302 inline 6 with a single central 4 barrel carburetor needs a richer fuel mixture at maximum torque peak than the carb will supply without some fussing with power valve restriction channels on the Holley.The Edelbrock or Carter wouldn't respond to metering rod or air flap adjustments.Finally I bumped into some tech info about fuel bleed holes in the secondary venturi clusters on some Carters.I drilled holes on a Carter 400 CFM 4 barrel on the GMC and it cleared up 95 percent of the bog and detonation.Supposedly all engines need a richer mixture at peak torque,then a less rich at it winds up to peak HP.
I believe many guys run too rich a fuel mixture on modified carburated inline engines and thus the complaints about poor fuel mileage,especially with GM 235-261's and GMC.I run street engines on the verge of detonation with lean mixtures ,aggressive spark timing,180-190 degree thermostats heated intakes and relatively high compression with as tight a quench as practical.Downside is any problems with fuel mixture curves show up instantly as heavy throttle detonation,it's a slippery dlope and you have to be careful.But the return is super sharp throttle response and good fuel mileage.And I don't drive with the throttle wide open for more than 10 seconds on the street.


70 Triumph 650 cc ECTA current record holder