when you move an app to sd, you copy the apk, basically, leave the rest of the files in the internal memory, and add some overhead for the parts that trick the system into believing the apk is in the right place. For some apps, that overhead is massive, as you can see.
Moving apps to SD was introduced in Froyo (or was it Gingerbread?), and it was a necessary feature when even top range phones had like 300 MBs of user facing storage (500-600 MBs of actual storage). Since then, the feature has remained in android, but it's usefulness has been decreasing, and the tradeoffs are not worth it in any midrange or better device.