Why member ring is favoured in chelates?

why chelates which form 3 or 4 member rings or 6 or 7 member rings are as stable as 5 member ring chelates? 
for example if we measure stability of chelates with oxygen donors, say oxalate, malonate, succinate etc oxalate chelated with metal shoes more stability. why malonate , succinate as chelating ligands are less stable?




In its **general** form, your statement on the stability of these complexes is wrong!


$\ce{Be(II)}$-malonato complexes are more stable than the corresponding oxalato complexes, cf. [*Preparative, potentiometric and NMR studies of the interaction of beryllium(II) with oxalate and malonate*](http://dx.doi.org/10.1016/S0020-1693(97)05520-5).


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