By thick I'm assuming he means the width. Common with Brembo alloy calipers that use stainless steel sliders on the inside that the pads move along. From experience, corrosion between the 2 metals form under the sliders which slightly lifts them off the surface, this results in it 'not seeming' like the pads will fit or, in your case, too wide (if that is the case). It's a tedious job splitting the calipers, removing the sliders to remove as much corrosion as possible & building them back up, I've had to do that in the past. At one point you were able to buy new sliders on ebay for the Brembo calipers.
Your issue may be completely different.