I came across a Japanese girl in Germany who talked using words like あたし instead of わたし, or ちっちゃい instead of ちいさい. When I asked her, she said that these words or usages of words are more common in Japan.
Atashi is for females. Chicchai can be seen as more emphatic "tiny". They are both quite common. Remember that in most textbooks for any language the most ideal form of the language (often judged so by prescriptive grammarians) is taught. As you familiarize yourself with real world usage (through travel, friends, and media) you will discover all sorts of words you were never taught because they seemed "too casual". DO pay attention because casual speech is appropriate for casual situations; nothing sets you apart as a gaijin more than using polite speech in every single conversation you have.
Yes, they're common, but those words in particular are very casual and あたし is only used by girls. Many women will not use it since it's so effeminate, but it's not uncommon.
あたし (atashi) is the female version of 私 for referring to yourself. わたし (watashi) is the neutral form for that, so you can use it always. あたし is only used by females, while 僕 (ぼく, boku) one of multiple possibilities for males is (although 僕 by itself is rather informal).
There are many words that are dependent on who it says, or who it is addressed. あたし is just one example there.
Only girls would use this kind of language, especially high school girls. If adults use it, it wouldn't sound right. You definatley couldn't use it within a business!
I think they use this kind of language to be kinda cute.
If you're learning the language, you should of course learn the proper words and maybe just look at these kind of feminisms to be aware of it to understand.
