If you mean the game data size - it's almost entirely object images. Sound also takes up a lot, but sound is streamed as you play the game so does not contribute to the loading time. 99% of the loading time is spend downloading images (Sprite animation frames and tiled background images). Everything else uses some data too, but it's so tiny in comparison you can ignore it (e.g. an event turns in to just a few bytes of javascript).
Construct 2 estimates your game's size - it's in the status bar as "Approx size". It's basically the size of all your images plus audio, so it's larger than the amount needed to start the game (which is just images), but it's a useful indicator.