Growing a raid 1 with mdadm

You cannot add partitions to a RAID1 and grow it. What happens is exactly what you describe; it added them as mirrors.

to grow the array, and when not using LVM (logical volume management), you need to grow the underlying partitions and then grow the array. This is most commonly done by removing a device from the array (set faulty with mdadm, then remove), repartitioning it, adding it back, waiting for resync, then doing it with the other. Then, you can use mdadm to grow it. The tricky part, though, is that with the new mdadm superblock, you can't just regrow the array any more. See this question of mine.

If you do have LVM and your original RAID partition is a physical device of your volume group, you can create a new RAID1 array with your new drives/partitions, make it into a physical device for LVM with pvcreate and then add it to the volume group. You can then use lvresize to resize the actual volume. There are plenty of howto's on the internet that tell you how, exactly.