It's not uncommon for people to compare their current partner with their ex, especially if the breakup is still fresh or if there are unresolved feelings involved. There could be several reasons why your ex is comparing you to his new partner:
Lingering emotions: Your ex might still have feelings for you or unresolved issues from the past relationship. This can cause them to make comparisons as they try to understand their emotions and figure out what they want in a partner.
Insecurity: Comparing you to his new girl could be a sign of insecurity on his part. He may be unsure about the new relationship and is seeking validation or reassurance by comparing it with what he had with you.
Idealization of the past: Sometimes, people tend to idealize past relationships, focusing on the positive aspects and overlooking the negatives. This might lead your ex to compare his current partner unfavorably to the memories of your past relationship.
Coping mechanism: Comparing you to his new girl could be his way of coping with the breakup. He might be trying to convince himself that he made the right decision or that he has upgraded to a better partner.
Seeking closure: Your ex might be trying to find closure from the past relationship by comparing it to the new one. This comparison could be an attempt to understand what went wrong or to reassure himself that he has moved on.
Wanting to evoke a reaction: Sometimes, people make comparisons to provoke a reaction from their ex-partner. They might be seeking attention or trying to see if you still have feelings for them.
Regardless of the reasons behind the comparisons, it's essential to focus on your own well-being and personal growth. If you find that his behavior is causing you distress, it might be best to set clear boundaries and limit contact until you both have had more time to heal and move on. Remember that it's not healthy to dwell on comparisons or live in the past; instead, focus on your own happiness and building a fulfilling life moving forward.