More weirdness - clip changes size when moved

Have you tried setting the proper lenth in the Inspector under track properties?

If there is no notes in the blank area then the clip will shrink to the last note.

The blank area you keep seeing is more of a visual thing than a real thing.

Technically the last note is the end of the clip.

I'm sure that didn't used to happen for me, lol! I must be going mad. 

Anyway, grrr. I wish it didn't. When I'm arranging EDM I like to see clips as blocks of even length, it helps as a visual guide when you're lining stuff up, if like me you don't like Aim Assist. 

I think if you stretch the clip as you want and then enable grove clip it will retain its length.

OK so now I'm confused. In the project I was working in, clips that I'd resized to a round figure were reverting back to their original length when moved, and soens pointed out that this is expected behavior. I thought that was strange, because I'd never noticed it before when doing exactly the same thing. I thought I was going crazy. 

So I just tried an experiment. I started a new project and inserted a MIDI track. I then dragged a Groove Monkee MIDI drum pattern into the track from the browser. It had a length of 7:944, which I resized to 8:000. Now, when I move it around, it retains its size. In fact I can make it 16:000 and it still retains its size when I move it. Sonar does not truncate it to cut off the empty space. 
What is going on? Confused!

Not sure why I said that either.

Whenever I Move or Copy/Paste a clip it retains it's size no matter where the data is.

I can even split off the blank area and move it around without altering it's size.

What's the point of having an empty clip? There's nothig there. Again, I think it is simply a visual reference.

So to answer your original Q of clips changing size when moved, I have no clue.

So I FINALLY worked out what was causing this annoying problem (for anyone who's interested). 

I had "non destructive MIDI editing" unchecked in the preferences. With this unchecked, MIDI clips will be truncated to the edge of the rightmost note in them after you move or copy them. So if you have a MIDI clip of size 7:840 and you drag that clip out to a size of 8:0, then as soon as you move or copy it, the size will be truncated back to 7:840. The manual's explanation of this setting is:

Select this option to make all MIDI editing non-destructive. When toggled on, this option forces deleted data to be retained even though it is not visible.

You can see what's going on in Sonar's clumsy way (sort of). When you stretch out a MIDI clip so that part of it is blank, Sonar sees this blank section as "deleted data" and retains it. Of course what it's doing is "retaining" data that wasn't even there in the first place - you've just fooled it into thinking that there is deleted data by stretching the clip out to include blank space. 
And I guess I didn't fully articulate why this is so important to me: I regularly stretch MIDI clips out to the nearest bar so that I can easily click on them and loop them. If you've got a MIDI drum clip that's 7:840 and you select it and shift-L, the loop points are going to be set to a width of 7:840 instead of 8:0, meaning the loop is going to be out of rhythm. Of course I could set it by drag selecting the bars on the timeline, but this is a pain. Anyway it doesn't matter now because I FINALLY got this sorted!