This is normal. The ‘timeline’ is actually a Storyboard. The size of the thumbnail is not a representation of the length of the clip.
If you click on the clip in the Storyboard you can find the length of the clip in the Player Window by looking at the timecode displayed for the IN and OUT markers. Additionally the speed of the Play-head will change while playing over clips of different lengths though. It will play faster over shorter clips and slower over longer clips.