I mean, if you look at the early episodes, Suzanne was decidedly svelte and slim.

https://qph.fs.quoracdn.net/main-qimg-e73fc80485e3dcbf03c0e2abe528746b

Eventually however, Delta's weight became so noticeable, that the writers actually wrote an episode called “They Shoot Fat Women, Don’t They?”

https://qph.fs.quoracdn.net/main-qimg-2b7ac213b5f931bc7bdd450fec710600

I think season 3 is when it starts becoming noticeable.

Agreed--there was a big change noticeable when S3 started--pun intended. I actually preferred fat Suzanne over thin Suzanne. She got funnier, especially in the last half of S4 when she had the bubble-cut hairstyle, not the modified page-boy like in the above pic.

the last half of S4 when she had the bubble-cut

Which I think is actually the most unattractive hairstyle on her, lol (but yes, some funny shows there!).

The exact midway point of season 2 is when the change begins. The episode "Getting To Know You Better" which was the first new episode of 1988.

I worked in a grocery store while going to college when this was on TV. I remember how the Enquirer put Delta on the cover constantly about her weight gain. I would see and stock the magazines. I would agree that season 3 it started. Her weight gain was like a national sensation. Every late night host had constant jokes. It was beyond ridiculous.