There are a lot of "biggest" locomotives

"Biggest" was used rather freely, and it really depends on what you're measuring.  Longest?  Heaviest?  Most powerful (indiacted horsepower, tractive effort, etc.)?  I'm pretty sure there were locomotives which exceeded the big Boy in every way that you can measure.  What I've been told, however, is that the Big Boy was the biggest all-around steam locomotive.

I suspect this will spark off some interesting and heated debate.  "Biggest" is somewhat akin to "best" and everybody has their own opinion on the subject.  I know a lot of N&W folks who feel the Y6b was bigger and better than the Big Boy.

Largest steam loco

You may want to check at:

http://www.steamlocomotive.com/misc/largest.shtml

there are statistic tables sorted by wheel, tractive effort, etc.

Largest in several categories was N&W's "Jawn Henry". 

The page "The 'Largest' Steam

The page "The 'Largest' Steam Locomotives" has moved.

Click here - http://www.steamlocomotive.com/misc/largest.php.

According to the information

According to the information they list, the C&O turbine outweighed the Jawn Henry by almost 40K pounds.

As in all things where comparisons are to be reliable and meaningful, we must decide on the meaning of terms first, or at least on what metrics we are going to use...and agree.

Biggest could mean 'gauge loading', or how much space they occupied in cross-section.  For example, what must the clear bore be of a tunnel, or the clearances on different style bridges, so that our "largest" locomotives can pass through safely?  How high should the catenary system be above the stacks of steamers or the stacks of caboose heaters?

Biggest could also mean longest, heaviest, widest...

Power is another thing.  Some locomotives produced more power than they could apply to the rails.  Some produced prodigious amounts of power early, but then ran out of boiler pressure.  The Erie/Virginian Triplexes were laden with that problem.  Their tractive effort at the outset was truly amazing, though.

Tractive effort?  No locomotive ever built could supply more tractive effort to the rail surfaces than their powered wheels could sustain with the weight on them.  Steamers were limited in that respect, unless they were of the 0-X-0 type (with no trucks) because the trucks were intended to distribute the weight above them on one, two, or three axles.  If they are supporting weight, the powered wheels aren't doing it all.  Diesels, as you would immediately understand, have tractive motors on all their axles...usually.  So, all axles are powered, but also each of them gets a portion of the total weight of the locomotive to apply to the rails....to stick to them.

Anyway, this could get long.  "Biggest" needs to be carefully defined.