The pipe that feeds my meter from the mains pipe in the road is 20mm. Currently the pipe from the meter to the house (about 40 meters) is also 20mm. My plumber is saying I should change the one from the Meter to the house to 35mm but my rusty physics says this would be pointless as the restriction would still exist with having a 20mm pipe from the mains to the input of the meter.

Anyone know the answer?

First off wha country are you from? Your metrics don't match up with our standard pipe sizes.

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My plumber is saying I should change the one from the Meter to the house to 35mm

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And this would accomplish what?
The only person receiving any benefit would be your plumber,
What did he tell you. That the increase in pipe size of the house service would give you more pressure? Why did this conversion come up at all? Were you having pressure or volume problems? How old is your house and what material are the pipes? Copper, Plastic or galvanized? What size is the house main once the service enters the house? MY advice? Unless there's something I'm missing, Save your money!
Good luck, Tom

Why was the plumber there to begin with, what issues are you having? Your rust physics did not fail you