Frieza really seemed to be in complete control of the situation. While Vegeta did have confidence, it was partially based on the idea that the team's powers would eventually grow great enough to win and that he'd become a Super Saiyan in the process; it was basically wishful thinking, to put it bluntly. Vegeta got carried away by telling Frieza to transform, given he couldn't have known how much stronger Frieza would get. Even if he made an assumption based on Zarbon's increase, that's incredibly risky since he's a completely different life form.
1st Form Frieza vs Vegeta = Frieza mid-diff
1st Form Frieza vs Vegeta, Gohan, and Krillin = Frieza mid/high-diff in most cases. The team have a puncher's chance if Krillin gets creative with his Destructo Disc or Gohan gets angry enough to boost his power