bilou wrote:
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My theory is that the portable receiver is matched for a high impedance
short antenna, where the ham receiver is matched for a 50 ohm antenna.

Your theory is absolutely correct :-)
A good example is the AM band car radio.
The great advantage of low impedance is that it is well suited to
coaxial cables.
In car radios there was an adjustable capacitor to compensate for the
short length of the cable and front end tuning was good only on the
station selected.

** But ( older) car radios had high impedance inputs on the AM band.
A short run of coax is a pure capacitor at AM frequencies.
The trim cap set the antenna coil to resonance while moving ferrite slugs did the actual tuning.

...... Phil