Why is it racist to call someone from pakistan a Paki but not racist to call someone from britain a brit?

i don't understand this should i feel upset and angry when someone calls me a brit? if so well i don't its cled as racist to call a person from pakistan a Paki but not racist to call someone from scotland a scot or someone from australia a Aussie its just short for the country were from even if someone says it with racist intent its not actually racist because its a short name for the country of origin so no harm done really and racists can live there sad hate filled lives without actually hurting you :) im british or brit you choose