K1 Legend has an interview with Overeem manager Bas Boon and they ask him straight up about Overeem's remarkable physical growth over the past few years:

K-1 LEGEND: Overeem’s move to heavyweight has seemed to really give his career a giant jump start, what prompted this decision and what went into his training to help him bulk up to being such a formidable heavyweight?

Bas Boon: We have talked a lot with Alistair about his lack of cardio in some fights when he fought in 205 lb class. He is a natural heavyweight, his weight then was around 102 kg (Alistair is 196cm) and he always had to loose about 10kg to make his weight, he would have problems later in the fight because Alistair’s aggressive fight style. We decided to move up in weight, from that moment on he fought Paul [Buentello] in Strikeforce 2007 for the belt (Alistair already defeated Belfort for the second time in a rematch in the states). He ko’d Paul in round two, then Alistair went through a difficult time in his private life which resulted in the loss against teammate Kharitonov, he did not have the good training before that fight as well his mental state was not good. We overcame that problem and the results are great, Alistair feels great, he does not gas anymore. Sparring in Breda with Zimmerman, Saki, Kharitonov, John Olav Einemo, he is improving by the day, his defence is getting better as well as his combinations and he can take a hell of a punch.

Boon also goes on in great detail about his business conflicts with Fedor Emelianenko's managers M-1 Global. If anything, the added details only muddy the waters. It sounds like Boon is backtracking a little bit from the way his earlier charges against them were presented in the online media. It's well worth a read. If anyone has the time to untangle the various back-and-forth and he-said-she-said I'd love to read it.