Spong is coming off of a loss to Alistair Overeem in the K-1 World Grand Prix last year while Mihaljevic is coming off of a dreadful loss to Catalin Morosanu. Spong has slowly moved up to heavyweight over the years until 2010 where he put on size fast. Spong says he has taken the time off between now and his last fight to put on size the right way and adjust his body to the change. Mihaljevic is a journeyman who mostly fights in Croatia.
I'll just start by saying this. Mihaljevic has no chance. If you've seen his fight against Catalin Morosanu, you can see why. (I don't recommend watching it, it's just dreadful to watch.) He's sloppy, has bad defense and reacts really badly to getting hit in the face. It's basically a squash match for Spong, who looks to get his 2011 started on a winning note.
Don't expect this to last very long. Mihaljevic is a pudgy Croatian journeyman who is being brought in to get smashed. There's not much else to say other than Spong will finish him in the first round.