Lady's Gaga's tour in Sofia, Bulgaria was originally to take place at the Vasil Levski National Stadium playing for 54,000 people. But due to low ticket sales, Gaga’s concert was transferred to a hall playing 15,000 - 17,000 people.
In Nice, France, the same story. Originally to take place at The Charles Erhmann stadium, has the capacity for 50,000 people. Same place where Madonna played with her MDMA tour in front of 30.000 people. Lady Gaga's concert will now take place at The Palais Nikaia, which has a 9,000 seat capacity. Tickets for the Stade de France concert are sold with a 70% discount. So don't believe the hype Lady Gaga is a sell out everywhere.

Have to say I was surprised to see Groupon (or was it one of the other companies?) offering her London ticckets at IIRC a 30% discount but let's be honest the venue is huge and the offer sold out quickly. She could never have been expected to be as popular as in 2009 because she's no longer 'new' and pop is a fickle business but fact is she's still flipping huge. I do still think the next album is the real decider, though. BTW sold HUGE numbers in its first week but didn't really sell consistently big (relatively) after that. Which no matter how it's spun must be a (slight) worry for Interscope. Of course, it still sold s sh*tload and if it hadn't been for '21' would have been seen as a massive success, I suppose.

I'm no fan of Lady Gaga but the OP definitely seems to be bashing her. I was at the Aviva a couple of months back to see Madonna and it's enormous. Madonna sold out most of it but there were a couple of tiers still empty. As mentioned above we're all skint in Ireland and to even sell out half of the Aviva is a huge achievement here.

The O2 is tiny in comparison. Most big artists play at least 2-3 dates in the O2 as it's always selling out.

It would be best to even do a bit of research OP before you make silly claims.