Why are my images shrinking when exporting mp4

[*Angel_Star*]

I created an animation in After Effects, the client was happy with it, it exported totally fine, none of the images shrunk. 

Then I made a copy of the file and then made a 2nd & 3rd animation, and saved them all in a new "Finals" folder, when I exported them as .mp4 now some of the images SHRINK (see screenshots below - the example with the larger grey boxes is how it should be, the smaller ones are after i've exported it).

And i have no idea what i have done wrong or how to fix it, and can't find anything on the internet for this exact search. 

please help me!

Specs: 1200x1200 animation. Exporting at mp4. 
i have screenshot my composition settings as well incase I've messed something up. 

Thank you in advance. 

[Mylenium]

Impossible to know anything since you haven't provided any details about how it looks on the timeline. From images having different aspect ratios to specific issues with hardware acceleration to the non-standard resolution of the output this could be anything. More info is definitely required.

Mylenium

[*Angel_Star*]

Ok, please bare with me as I am a beginner and just learning how to use After Effects....

1. The animation did export totally fine from my orginal file with the exact same settings, but seems to not now I have moved the .aep file, i'm not sure if this has anything to do with it? 

2. I have included a screen shot of the exact point in the timeline, with the timeline items expanded also... there are no settings in the timeline that shrink the images... They appear perfectly as they should when viewing in After Effects, it is only at the export stage that they shrink.

3. Its also worth noting if I export to Premier Pro, I can open the file there and the images also appear correct size. 

What other information might help to get to the bottom of this issue?

Thanks

Angie