Loafers can be difficult to judge because their heel fit is connected to the width and height of the front. A pair that grips the heel may press across the instep; a comfortable front may leave the back moving excessively. The answer is not always a smaller size.
Try both shoes with your intended socks or hosiery. If you plan to wear them without visible socks, use the exact low-cut pair you expect to keep in the rotation. Sock thickness and grip can change the result enough that a store try-on with a different sock is misleading.
Compare the whole foot, not just the heel
Stand and check room at the toes and across the forefoot. The upper should not create painful pressure merely to hold the back in place. A loafer's vamp—the part covering the top of the foot—can have different heights and lengths across models. That shape may be the key variable.
Walk, turn, and pause. Notice whether the heel lifts slightly with the shoe's movement or repeatedly rubs and threatens to come out. Avoid a rigid rule that every trace of movement is unacceptable, but do not dismiss substantial slipping as something that will certainly disappear with wear.
If a smaller size secures the heel while crowding the front, compare another width or model. If a larger size relieves the top but leaves the whole foot sliding, the same conclusion applies. A different last can offer a more suitable combination without forcing a compromise between two uncomfortable options.
Do not budget on an untested fix
Heel grips, tongue pads, and insoles alter the interior. They can be useful in some situations, but they also change where the foot sits and how much space remains. Include any intended accessory in a careful fitting rather than buying the shoe first and assuming a pad will solve it later.
Inspect the return conditions and keep the soles clean during the decision. Softening with wear is not a promise that a shoe will become the right size or shape. The best loafer is one you can walk in naturally from the start, with enough room at the front and a heel that feels acceptably secure to you.
