iTunes Enquiry

Hi I managed to get all of the music from my computer onto my new laptop but now I have a problem getting the music on my phone. I tried the erase and sync option but most of the music on my phone from my library on the computer is taking up the space on my phone. I don't understand why the music won't erase first and leave space to sync with the iTunes on my laptops library. Help would be appreciated! I've tried everything to delete apps etc to leave as much space as possible for the new iTunes but the iTunes on my laptop is the exact same as the one on my computer I would just prefer to have it synced with my phone. The problem is mostly the space but its syncing mostly all the same songs only a few new ones so i don't understand why so much space is required! please help! Thank you.