Summoning rabbits without inputting a spawn event will spawn a normally sized rabbit, but if you add any spawn event, even just a random string of letters, it creates a large rabbit. For example,

/summon rabbit ~ ~ ~ sdhiausdhiu

will summon a rabbit that is oversized. The "sdhiausdhiu" can be replaced with any word, or spawn event, such as making it snowy, or with a desert skin. Any spawn event will cause the rabbit to be larger than normal, though.