The Gemara interprets the word ‘Gadol’ in reference to ‘Onesh’ (greater liability) since laws of Shabbos apply also to things detached etc.; why is this related to as a ‘liability’ and not simply another detail present in Hilchos Shabbos and absent by Shemittah?
How do we understand the obligation (according to Rav and Shmuel) of a Korban Chatas for a ‘Tinok Shenishbah’ who had no prior knowledge or awareness of this mitzvah; why is he not regarded as a Ones?
The Issur is greater, since it is prohibiting more objects. Every Issur binds an object from a person; it prohibits a person from something. Shabbos applies to more objects so it is a bigger Issur.
We find this concept when Chazal say that it can’t be that we would have something that is prohibited to a non-Jew and permitted for a Jew, while we can find that the Onesh for a non-Jew would be more harsh than that of a Jew. If the idea is that the Torah is first and foremost for us, then that should apply for Onshim as well. So we see that the point is more that an Issur means that we are Makdish the object to Hashem, we are giving it up to Hashem. Therefore, it can’t be that a non-Jew gives more to Hashem than us, who were chosen to be an Am Kadosh.
Technically you might be able to answer that although he is a Tinok Shenishba and had no access to proper information, it is possible for him to understand that since he was never exposed to the main body of Yiddidhkeit he should go out and find it. However, there is a better explanation:
An Onnes is when somebody’s actions are forced, and does not reflect on any shortcoming of his. A Shogeg, is when although not faulted, a Kapara is necessary because the Cheit originated though a human shortcoming. Not knowing that the day is Shabbos is not anybody’s fault. We can hardly tell him you should have asked if today is Shabbos. Nevertheless, forgetting and not realizing are human shortcomings, and since we try to match up to higher standards we need a Kapara.