Caro, firstly try not to worry. The corncob that Fern ate will probably not cause a problem, and I wouldn't have thought having a wider ribcage should be any detriment to her.
What happened with Billy was in the July he stole a corncob out of the dustbin. I wasn't 100% certain at the time that he had eaten it, but a few days later Billy threw up a part of a corncob. Several days later we were at a show, and Billy, not normally a sickly dog, had a very runny and quite explosive poo, in which he produced another piece of corncob. I was a concerned but as he had no further ill effects I thought no more of it, having no prior knowledge of the potential problems.
Over 4 months later, Billy started throwing up one day. I kept my eye on him, as you do, then he was sick a couple of hours after eating, and over the next 24 hours it had got to the point where a sip of water was making him sick. I forgot to mention (actually making it 4 pieces in total) he brought up another piece of corncob. We had a night with him throwing up and being quite distressed, (I would have taken him to the emergency vet but the one on shift told me it could wait till morning)
Anyway, to cut a long story short, he was in the vets a couple of days having x-rays etc before they opened him up and found a piece of corncob that, hey presto I had told them would be in there from the beginning.

However with hindsight, he had been showing symptoms that there was something wrong, which we had wrongly put down to him being mardy as it had always seemed to coincide with my hubby going back to work away from home each week (daddy's boy!!). He had been sitting in the garden under the bush and shuddering, not wanting to come in, he also had intermittent diarrhoea, but just as equally had perfectly fine days, so it never occurred to me that it was all connected at the time.
Just keep a check on Fern, eating and pooing etc It would be very unlucky for it to happen, but just be aware that it can, and banish those corncobs from greedy cocker tummies!