Imagine a group of 3 stars that appear to be equally luminous ,star A, B, and C in the shape of an equilateral triangle. A,B and C look like they form an equilateral triangle right now on the Earth but when the light from star B reaches the Earth, the light from star C still has say, 10 years of traveling to go. So when B is observed in its present position, the apparent position of C is from 10 years in the past. Where C is now relative to the present position of B might be sufficiently different so as to not to appear to form an equilateral triangle. The 3 stars 'right now ' could form a 'scalene triangle' pattern if star C's position ,where it is 'right now', were visable . If you wanted to 'imagine' the 'smallest' sphere 'covering these 3 stars 'right now' , what would be it's radius? ( given star A is 100 light years away, star B is 110 light years away and star C is 120 light years away.)
The shapes of constellations (and there are several different depictions of any particular constellation) only depend on how they look at any specific time. Stars in any constellation are not necessarily close to each other in space. For example, the main stars in Ursa Major vary in distance from 58 to 124 light years.
The boundaries of constellations are determined by the International Astronomical Union, and are not set by reference to the positions of the stars.