My reasoning is as folows, but maybe, I am wrong. I am not that god in theory.

Capacity of suction line HE depend on TD of that heat exchanger. If we have low discharge temperature, than TD of suction line HE is lowered, and therefore achieved superheat/subcooling is also lowered.

Nominal SLHE selections should be based on system horsepower, which will provide a nominal 10ºF subcooling and a nominal 20ºF suction superheat for R-22, R-502, R-404A, R-507, R-407A, R-504, R-134A, R-407C and R-410A at 110ºF condensing and 25ºF suction. For example, for a 1-1/2 HP system, select an SLHE 1-1/2. Do not select undersized models to avoid high suction pressure drops and gas whistling.

Selections based on suction and liquid line sizes are appropriate for standard low, medium and high temperature refrigeration systems provided the system HP rating closely matches the SLHE selection.

Selection of oversized models will provide additional liquid subcooling and suction superheat. Oversized models are acceptable only if the suction gas temperature to the compressor is not raised above 65ºF for proper compressor performance.
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