Formal structure of  "Back in the USSR":
Verse 1 0:16-0:30
Verse 2 0:40-0:53
Chorus (ext) 0:53-1:06
Middle 8 1:06-1:23
Solo (v) 1:23-1:36
Middle 8 1:46-2:03
Verse 3 2:03-2:17
Coda (m8) 2:27-2:43
Comments: The second chorus is extended by repeating the words "back in the US" twice before completing the phrase ("back in the USSR"). This elongates the duration of the chorus by three seconds (13 seconds, compared to the 10 seconds of the other four choruses).
Given that the solo functions as a verse (the solo instrument replaces the lead vocal, but everything else remains the same as heard in the verses), the macro-scale formal design can be seen as four iterations of verse + chorus + middle 8, with the first such iteration omitting the middle 8:
Lastly, in following this pattern of verse, chorus, middle 8, the coda is based on the middle 8, which is quite rare in Beatles music - something only used twice before in  "All My Loving", and  "In My Life".
This blog is a workshop for developing my analyses of The Beatles' music.