The memory-size was always just a value to base other config from - it was never a hard cap on memory usage. This was a somewhat common point of confusion. The memory governed by configs based on memory-size would necessarily change due to the major changes in how data-in-memory is done in 7.0. Since the memory-size the definition was going to drastically change, we opted to rename it to indexes-memory-budget, we think the new name is more intuitive given the reduced scope.
I recommend reading the 7.0 announcement for more information about these changes. Let me know if you have any further questions about that.