I understand MRIs are largely a compromise. Organizations will buy only 1 or 2, and therefore buy one that will fit all but the most obese patients, to the imaging detriment of anyone/anything smaller. Or is that all wrong?
The wider bore does indeed come at a cost. You have more geometric distortion and a less homogenous magnetic field at the edges of the bore. Also, you will need stronger magnetic gradients to form image, causing more energy disposition (i.e. Heating the patient) and higher probability of peripheral nerve stimulation (PNS, involuntary muscle twitches due electric fields induced in the nerve caused by MR gradients).
To get a good high resolution signal you need field homogeneity (and/or accurate mapping), high quality gradients, and good SNR. Field strength helps with the last part.
There are other trade offs too, for example imaging artifacts due to implants (or even being able to image them), susceptibility, etc. vary with main field strength.
It always confused me that MRI did millimeter scale imaging with radio waves that are meters long. I learned, and other's might be interested to learn, the trick is that it's the magnetic gradient that does the imaging. Each nucleus in the volume is tagged to it's place along the magnetic gradient axis by the local magnetic field strength shifting the emission frequency proportionally. That frequency shift is then inverted back to position for that axis.