First thing, when you want to export it your desired format(for example : H.264 format,H.264 is also known as MPEG-4 AVC. Developed for use in high definition systems such as HDTV, Blu-ray and for low resolution devices) you have to customize the export settings.
The two factors which determine file size are program duration and bitrate. The longer your program is, the larger the file will be.
But the bitrate is, Choosing Uncompressed will have the highest bitrate possible. Choosing an MPEG variant (MPEG1, 2 or H.264) will allow you to lower the bitrate (and hence create a smaller file) while still achieving acceptable quality.
Under the export settings you are getting the Preset.You can select a preset the way you want it.But keep in mind choosing the correct Preset with the FPS(Frames per Second).Choosing a lower FPS would reduce the quality of the video.
In this issue it's recommended to use the resolution of the Original video(which is 720p) in the preset.You can change the FPS and check for the quality.Here is how you check the FPS of a video in Premiere Pro.
Choose the exact resolution if you want to get the same file size Under the video tab in the export window.Change the level (which constrains encoding parameters bitrate range and FPS)and you can get the actual file size by changing it.You will see the estimated file size below. This is not recommended as it reduces the quality.
You can customize the export settings same as the original and you will get the file size you mentioned which is 27MB.That's how you got 27MB for the file size.
Note: Codecs do the magic!.