When you originally edit the HD video into the incorrect (undesired) frame size timeline, FCP Scaled the clips to fit. When you copy them from the incorrect timeline, and paste them into the new one, this incorrect scaling is retained.
That is why you want to use Remove Attributes - to remove this incorrect scaling.
If you double click on one of your now "even smaller" clips on the new HD timeline you created, it will open in the viewer. If you click on the Motion Tab, you will see that it is scaled down from original size. The original size is 100% Scale.
If you Control Click on that clip in the timeline, and choose Remove Attributes, you will be given the opportunity to check the box for Basic Motion (of which Scaling is a part) and you can remove it there. That will reset the scaling for the clip to default - 100%.
By following the instructions in the other thread, you can select a group of clips, or all of the clips in the timeline, and by Control Clicking on any one of the selected clips, remove the Attributes for all the selected clips.
MtD