UIImageView in my UITableViewCell changes size on click

I have a fairly vanilla UITableView in my app used to display an image and text in the default/standard way. The first time a UITableViewCell is requested I return it with a placeholder image and start an asynchronous download of the real image. When the download is complete I replace the placeholder image.

If a download fails, the placeholder image remains. Clicking on such a row acts normally. If the download is successful, clicking on a row with the intended image leads to the UIImageView expanding to the height of the row (and the width increases at scale). The UIImageView never returns to normal size. Crude attempts to set the frame of the UIImageView do not alleviate the issue of mysterious resizing.

// from my - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath; method
// after boilerplate cell dequeuing code
cell.textLabel.text = [selectedNoun title];

// add placeholder image
UIImage *placeholderImage = [UIImage imageNamed:@"placeholder.png"];
cell.imageView.image = placeholderImage;
[cell.imageView setContentMode:UIViewContentModeScaleAspectFit];
//...
// If the image has been downloaded I set it
cell.imageView.image = iconDownloader.image;

This is driving me nuts. It actually only happens in one of two tables in my app, but after comparing them line or line I can't see any difference. Hoping that someone has come across this before.

Thanks!

EDIT: I don't have a good explanation for my solution other than to say that images over a certain size appear to lead to this behavior and the use of actual thumbnails (even images somewhat bigger than the UIImageView's frame) do not exhibit this behavior.