Merging projects is not supported in GarageBand.
You can however try to open the smaller of the two projects and retrieve your recordings from the package and recreate the tracks in the other project. There is a nice description on Bullets&Bones:
I've been able to copy tracks from one project to another no problem .
Exp : I like a track from Project A . With A open ... I copy the track ( command C ) I want to move . With Project A still open ..... I select project B . Project A closes and project B opens. I then select where I want the track to go in Project B . Then I hit command V .
This works for me .
I'm pretty simple minded that's why I'm level 1 and ask more questions than I chime in on.
I did a test to make sure I wasn't handing out bad info .
First there was No automation used on the track from project A I wanted to copy . Because I am a byte or two simple as times , I'm not certain what you mean by regions . Enlighten me on that please .
Here's what I did. Blow by blow .
From A I selected a bass guitar part . The whole track turned green . The track was built up from smaller sections of 8 bars. Track length 128 bars. This was something I already had .
For Project B where I want to copy to . I made a blank start for a tune . I made one track for piano or midi GB sounds as I've always got it . Left this track named "Paino" .( I've also copyed parts & whole tracks to projects in progess ) .
With A open I copied the the bass tk I wanted to copy ... then with A still open ... I hit B. A closed and B opened .
I copied ( command V ) to the the piano tk . ( only thing there ) .
It copied the whole track . All 128 bars . But it was a piano & not a bass. So I picked a bass sound. It had done everything I was wanting .
This works for me . There may be better ways ... I don't know them .... yet anyway . But this should work fine
That is a perfect description of what HangTime was referring to, when he wrote:
but you can copy the regions within tracks
Your "smaller sections of 8 bars" are regions in a track - you selected all and copied them all to a different track in the new project. But it is only a part of the track - some attributes of the track cannot be copied. You have to recreate all track settings - the instrument for one, change "Piano" to Bass", add the effects again, the panning, the volume, the automation curves. Essentially, you can copy the audio or midi for the track, but have to do all the work again, that you invested into the mixing of the track with other tracks.