Generally, we identify ourselves with body-mind and the Atman/Self is not known to us. Also, for spiritual quest we are advised to look within. These considerations lead us to believe that if Atman is there it is something very small inside the body. If we look at Panchakoshas concept explained in Vedanta this misunderstanding can be corrected. There are 5 shells and Atman is beyond these shells. The innermost shell is Annamayakosha (body), then there is Pranamayakosha (breath/energy), the next one is Manomayakosha (mind), the penultimate one is Vijnanmayakosha (intellect) and the outermost one is Anandamayakosha (bliss). Atman is beyond Anandamayakosha.