Transformer question (puzzled)

Honeywell's published documentation for their R182C switching relay contains the following warning:
IMPORTANT: The transformer [120VAC primary, 24VAC secondary] on the R182C may overheat when used with a series 20 thermostat if the total resistance of the thermostat circuit exceeds 2.5 ohms. If the measured resistance of the thermostat (including thermostat wire and thermostat contact resistance) exceeds 2.5 ohms, add a 100 ohm, 10 watt resistor between the W and R terminals.
I don't understand that -- if the total resistance is, say, 10 ohms, clearly the current drawn will be *lower* than it would be at 2.5 ohms, or at 1 ohm.
How is the transformer in danger of overheating at a *lower* current? Seems to me that the greatest danger of overheating would occur with a circuit resistance of near zero ohms, i.e. a dead short across the transformer secondaries. Someone please explain this to me.
[Please note that I *do* understand the purpose of the 100 ohm 10W shunt resistor, in reducing the total resistance of the connected load. What I *don't* understand is how a *lower* resistance avoids overheating the transformer.]
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