I would like to know if it's common to have a thin sounding 1st string on a strat.

The output volume from the string is lower if it's checked directly from the guitar's ouput jack plug.
The output can be seen to be much lower by measuring it (using an oscilloscope), or checking it's level into a recording desk. The output is about a third lower.
But it might not only be the volume effect , maybe it's the tone of the string itself?

The G or 3rd string is really much louder than the others. I'm not sure if this is caused by the radius of the finger board, and if it's a normal thing.

I have a Kahler tremelo. The strat is made in Japan in 1989.

Cleaned the Kahler , so that the wheels are free. Adjusted my action higher to allow for more pick attack. A new nut was fitted by a guitar smith. He changed the 1st string to an .011. This did balance the sound better, but i don't like the feel of having different tensions on the strings, so went back to a .010.

Iv'e been forced to compensate with either gluing metal nuts onto the pick up poles, and also fitting a hum bucker with adjustable screws. This does help a lot. I'm still not entirely happy with the sound.

Wondering if anyone else has this problem?

Welcome.

If the contour radius of your saddles does not mimic that of your fingerboard, this can be a contributing factor to your problem.

The G string is the loudest because of the pickup's polepiece stagger. See, the original design of the pickup was for a wound G string where all string sets had one; we're going back to a late 1940s / early 1950s design. Due to magnetic properties, a wound strings doesn't 'pick up' as easily as a plain string and the thinner any string is in general, the harder it is to 'pick up'. It's simply a physical mass thing. As you may now surmise, the G string's polepiece would be then be the highest of the six. Consequently, using a plain G there which is now the thickest of the plain ones will really make it jump out, volume wise.

FYI: Unwound G strings didn't start appearing until the late 1960s yet Fender and 'purists' resisted a pickup design with the G polepiece at least as low as the B string. For CBS (the then Fender), it would involve retooling ($$$) and for the 'purest', it would alter the magnetic field, thereby altering the tonality of the pickup as a whole for better or worse.

Many players complain chiefly about the B string rather than the high E being of too low a volume. However, keeping in mind the above where the high E is significantly weaker than the rest (excluding the G, of course), Essentially there are several possible root causes. It could be one, all or any combination thereof:

1. As you've cited, the polepiece may very well not be high enough.
2. The string isn't fully over its polepiece.
3. Somehow the magnetism of that polepiece has petered down.
4. The Kahler and/or the nut isn't efficiently transferring the E string's vibration in comparison with the others. Or to put it another way, there is a microscopic air gap somewhere causing a premature decay of the string.

You can rule out the inherent magnetic properties of your high E being different, premising you are using a complete, same brand string set.

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