HTML5 Build is MASSIVE - how can I reduce its size?

Just created my first game in Unity! I'd be rejoicing if the build wasn't so big. It's a simple 3-level game but it's 50MB in size. I tried hosting it on my server but (obviously) when you visit the page, the player downloads ALL The assets before launching the game. That's a 3 to 10 minute wait depending on the speed of your internet connection. Is there anything I can do to resolve this?

Apologies if this is a frequently asked question - I've been searching high and low and been unable to arrive at a solution.