If Tau are the weakest race, why can't anyone beat them?

It's a lot easier to defend a small area.

Tau are just more efficient. The Imperium is massive but is wasteful and slow to react. Plus the Tau are motivated versus press ganged Guardsmen treated as cannon fodder. Sure, in another 20k years, the Tau probably will go through their own Men of Iron revolt but right now they're in their expansion phase.

Tau tend to lose against eldar, and Orks give them a pretty rough time as well. I don't think the Tau have ever been anything but BTFO by the Necrons.

Ghazghkhull and his Bullyboyz BTFO'd an entire Tau colony that had a lot experience fighting Orks in 704999.M41, as I recall.

Funny, for all the taus bullshit, they seem to suffer most under the Orks

Because the moment their plot armour drops this happens.

Fourth Sphere of Expansion got inside a Warp fissure and will probably be all over the galaxy. This will allow hte T'au to be in more campaigns, but also allow more catastrophic losses so long it affects the faraway colonies but not the Empire itself.

Meanwhile the Fifth Sphere of Expansion has enlarged the T'au Empire to an extend that you can now see it without zooming in. It has been a full century since Cadia.

I love the Tau, but making them so small was a mistake. If the Imperium loses a planet it's not a huge deal, if the Tau lose a planet it's a massive blow. The Tau essentially cannot suffer defeats on the same scale as the other factions, or else it cripples them as an effective force. So they're essentially forced into always winning crucial engagements, because if they don't they're gone entirely.

But user, Chaos has been deprived of attention for so long. The 4th Sphere Expansion will turn chaos so that Chaos can finally, at long last, appear in the fluff again.

Nah. The T'au have evolved/selectively breed and socially engineered to be resilient to Chaos corruption.