Minecraft wiki entry for Development_versions, associated text: Replaced  shrunk with  coordinate_scale.
For example,  coordinate_scale for minecraft:overworld is 1.0, for minecraft:the_nether is 8.0, thus the /execute in command to go to the Overworld actually multiplies the execution position (only the X/Z part) with 8.0/1.0 (1.0/8.0 when vice versa), which is 8 (0.125 when vice versa).
If there are two custom dimension type custom:a with coordinate_scale set to 2.33, type custom:b set to 3.14, then the /execute in command to go to custom:a would multiply the execution position (only the X/Z part) with 3.14/2.33 (2.33/3.14 when vice versa).