[Al(H2O)3(OH)3]

Why is [Al(H2O)3(OH)3] not fully hydrolysed to [Al(OH)6]3- in excess NaOH?
For my exam board AQA, it says the chromium hydroxide ppt dissolves to form [Cr(OH)6]3-, but not for aluminium, which forms [Al(OH)4]-.

My initial guess would be that since Al^3+ is very small and OH^- is charged, fitting 6 OH^- ions around the aluminium would be very difficult due to the overwhelming electrostatic repulsion.
A similar explanation would probably also suffice for the fact that [Al(OH)5]^2- isn’t formed.