Replay analysis: Why opponent got bigger army and why psy storm is so weak?

Recently I played PvZ and lost (http://ggtracker.com/matches/5047035). Trying to analyze reasons, I've figured out that at the beginning of the 1st big engagement (around 16-30) opponent had army which had 40 more supply than main.

I analyzed replay and cannot understand how did that happen.

I do see mistakes from my side:

• non-used resources (but opponent had poor resources usage as well)
• non warping units during the battle (neither opponent did)
• etc...

But still, if opponent has such a big advantage in quantity, it is hard to beat him.

Question: what did I do wrong so I had so smaller army?

Side question: looks like psi-storm is not really effective vs hydra/roaches... should I better start with Colossus?

Thank you for any advise!