Info-scroll-prefer-subnodes is a variable defined in `info.el'.
Its value is

If non-nil, SPC in a menu visits subnodes.

If this is non-nil, and you scroll far enough in a node that its menu
appears on the screen, the next SPC
moves to a subnode indicated by the following menu item. This means
that you visit a subnode before getting to the end of the menu.

Setting this option to nil results in behavior similar to the stand-alone
Info reader program, which visits the first subnode from the menu only
when you hit the end of the current node.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.