Extended sizes in American menswear are usually sold in a separate department, a separate store or a separate website. That separation is a production decision as much as a merchandising one.
Grading fails past a certain point
Standard sizes are produced by grading, which expands a base pattern by fixed increments at defined points. The method assumes the body's proportions scale together.
Beyond a certain range, they stop doing so. Chest circumference grows faster than shoulder width, and the posture and front-to-back balance of the garment change.
A garment graded past that boundary hangs wrong regardless of measurement accuracy. Extended sizing therefore requires a new base pattern, developed on a different fit model.
Tall and big are separate variables
Length and girth are independent, so a range covering both needs a matrix rather than a line. A retailer must decide which combinations to stock and which to omit.
Every combination stocked is another item to forecast, and the least common combinations sell slowly. The tails of the matrix are where markdowns accumulate.
Most retailers respond by carrying the dense middle and leaving the edges to specialists. That is why the range is broad in some combinations and absent in others.
Fabric consumption changes the cost base
Larger garments use more material, and the cutting layout becomes less efficient because fewer pieces fit on the same width of cloth.
Manufacturers price this in, which is why extended sizes sometimes carry a surcharge. It is a real cost rather than a penalty, though it is rarely explained.
Some brands absorb it into a single price across the range, which quietly means the smaller sizes subsidize the larger. Both approaches exist in the market.
Mixed rails do not work in stores
Placing extended sizes on the same rail as regular sizes makes both harder to shop, since a customer sorting for size faces a much longer rail.
Separating them improves the experience for everyone but creates a visible division that some customers dislike. Retailers have argued about this tradeoff for decades.
Online stores can present a filtered view instead, which is one reason the specialist business moved online faster than most menswear categories.
Specialists win on depth
A store built entirely around extended sizing shares one pattern block across its whole assortment and never competes internally for floor space.
That allows depth in the combinations a general retailer abandons, and it allows staff who understand the fit issues to advise on them.
The trade is range and price competition, since the customer base is smaller and buying power is lower. Depth of fit is bought with narrower choice.