So whether it is the sin of fornication or lying isn't it all the same since we don't classify any sins bigger then others?
This is not exact. There is no detailed classification as in catholic church between mortal and non mortal sins. However, there is a gradation in sins as attested by differents elements.
1) The penance varies according to the gravity of the sin as and is a clear indication fo its gravity. For instance, according to the canons of Saint John the Faster, having sexual thoughts that should not be, is penanced with 12 prostrations but committing fornication is penanced with 2 years without communion, fasting until the 9th hour and 250 prostrations a day.
2) The concept of mortal sin does exist in orthodoxy and the word is used by Saint Nikodemos of the Holy Mountain that explains that other used it before him without giving a accurate definition: for some mortal sins were sins condemned by death in the Old covenant
But at the same time, as Jean-Claude Larchet states it in his book "La vie sacramentelle", the final consequence of a sin is not known. What seems to be a small sin might have very consequences... You slander someone, the rummor grows and he ends with his reputation ruined, his job lost, a mental depression and commits suicide. The example is mine.