Dengys tystiolaeth ac ymchwil ryngwladol fod cysylltiad cadarnhaol rhwng dosbarthiadau llai o faint a chyrhaeddiad, yn enwedig ar gyfer disgyblion o gefndiroedd tlotach. Mae hyn i'w weld fwyaf ymhlith plant iau. Yn ogystal, mae lleihau maint dosbarthiadau yn cael yr effaith fwyaf o ran codi safonau pan gyfunir hynny â newidiadau a diwygiadau i addysgu ac addysgeg. Mewn ymateb i'r dystiolaeth hon, mae Llywodraeth Cymru wedi darparu cyfanswm o £36m dros dymor y Cynulliad hwn i leihau maint dosbarthiadau babanod, gan ddechrau gyda'r dosbarthiadau mwyaf gyntaf.
International research and evidence tells us there is a positive connection between smaller class sizes and attainment, particularly for pupils from poorer backgrounds.  This is most significant for younger children. And a reduction has the largest effect in raising standards when accompanied by changes and reforms to teaching and pedagogy.  In response to this evidence, the Welsh Government has made available a total of £36m over the course of this Assembly to reduce infant class sizes, starting with the largest classes first.