The pasuk states (Tehillim 38:19): "For I relate my iniquity (avoni); I worry about my sin (chatasi)." Sefer Eretz Hachaim (page 120) quotes Rav Moshe of Savrin zy"a as explaining that there are some people who, unfortunately, commit numerous aveiros every day, such as lashon hara, speaking falsehoods, getting angry, etc. They do not feel true regret but simply say offhandedly, "I spoke lashon hara today..." They speak about it as if it was a small thing.
There are other sins, however, that one is very concerned about, even if only done accidentally. For example, if one inadvertently desecrates Shabbos by carrying a handkerchief on public property where there is no eruv, he feels terrible about it.
Accordingly, the pasuk is speaking of someone who merely "relates" his avon ("small" sin), in an offhanded way, but worries greatly about his "chatas" ("big" sin). In truth, however, one should truly regret every transgression.