Ive noticed that even when I reduce the video size to 1080 / 30FPS a 2.5 minute video is close to a gigabyte...  is this normal?  A similar video taken in my iPhone in the same format is much smaller.

The resolution of the video is not the only thing to determine size. Video optimized for editing is going to take up more space. Once you have edited the video, you can save it in a format that is better for viewing normally. One of the tricks, for example, used to make videos smaller is to use key frames and then only update parts of the image that have changed. Highly compressed videos have fewer key frames. This is not good for editing. When you edit, you jump around the video, maybe reverse some clips. It’s more difficult for a computer to deal with this with fewer key frames. The Mavic Air and Mavic 2 Pro have a pretty high bit rate for recording. I would expect comparable files to be larger compared to videos recorded on a phone.

The bit rates will be different as per post number 2.
Right click on each of the files you are comparing and look at Properties/Details and you will see the bitrate used when creating the file. The higher the bitrate the more space it takes up.