Is every innovation more severe and grave than every major sin, or is there some detail to this statement?

Not every innovation (bid'ah) is greater than every major sin (kabirah). While innovations are originally more severe and graver than major sins, a sin might be greater in some cases. Innovation is a sin and more; it can be difficult to counter once it spreads because it is often wrapped in emotional appeal. The category of innovations is more heinous and graver than the category of major sins because innovation is an opposition to the Prophet (peace and blessings be upon him), an attempt to rectify his teaching, and legislation for a religion that Allah did not permit. It involves worshipping Allah through things that were not part of his (the Prophet's) Sunnah, in terms of belief and specious arguments. One should not downplay the significance of sins on the pretext that they are less severe than innovations; rather, it is obligatory to guard against both.