I doubt it was replaced.
It would be a minimum of 3/4" and more likely 1" or more for that distance. My guess is that the line broke outside the foundation and some DIYer or the home owner made their own repair using 1/2" copper rather than replacing the whole line due to the cost. On top of that your original line is most likely galvanized steel that has coroded inside the line so the pressure is reduced because of that. This is just a guess.
No plumber worth anything would use 1/2" to feed a house or repair a line unless it was 1/2" to start with which I highly doubt.