In general, Eastern Theology avoids the term, but the concept of differing severity of sins does exist. The rigid hierarchy of sins espoused by some Roman theologians (and the doctrinal approach which was put forward with the doctrine of Indulgences) is alien to the continuum of sinful acts which pervades at least the Slavic traditions. Some are known to be minor, some major, but it’s not so cut and dried.
And as Father Deacon implied above, missing liturgy does nothing to help one’s relationship with God. But it is not defined in terms of Obligation in the canon law of the Eastern Catholics.