I'd say mess with SetTexCoord until it looks ok:
Code:
local myButton = CreateFrame("Button", nil, myParent)
myButton:SetNormalAtlas("QuestCollapse-Show-Up")
myButton:SetPushedTexture("Interface\\Buttons\\UI-SpellbookIcon-NextPage-Down")
myButton:GetPushedTexture():SetTexCoord(0.1, 0.9, 0.1, 0.9)
myButton:SetSize(25, 25)