The attached patch for the parser reduces "Maximum resident set size (kbytes)" threefold, for the degenerate example of 'import ast; ast.parse("0,"*1000000, mode="eval")', by eliminating many CST nodes that have a single child.
According to the comment in node.c -- "89% of PyObject_REALLOC calls in PyNode_AddChild passed 1 for the size" -- the memory saving should be generally applicable, and not limited just to this degenerate case.
Modules/parsermodule.c is not yet updated to match. Please tell if you want me to do that, in case that my proposed change to the parser is acceptable.