We perform a detailed analysis of the reversible magnetization data of Salem-Sugui et al. and Babíc et al. of underdoped and optimally doped

Y {Ba}_{2} {Cu}_{3} O_{7 - δ}

single crystals. Near the zero field transition temperature we observe extended consistency with the properties of the three-dimensional

X Y

universality class, even though the attained critical regime is limited by an inhomogeneity induced finite size effect. Nevertheless, as

T_{c}

falls from

93.5 to 41.5 K

, the critical amplitude of the in-plane correlation length

ξ_{a b 0}

, the anisotropy

γ = ξ_{a b 0} ∕ ξ_{c 0}

and the critical amplitude of the in-plane penetration depth

λ_{a b 0}

increase substantially, while the critical amplitude of the

c

-axis correlation length

ξ_{c 0}

does not change much. As a consequence, the correlation volume

V_{corr}^{-}

increases and the critical amplitude of the specific heat singularity

A^{-}

decreases dramatically, while the rise of

λ_{a b 0}

reflects the behavior of the zero temperature counterpart. Conversely, although

ξ_{a b 0}

and

λ_{a b 0}

increase with reduced

T_{c}

, the ratio

λ_{a b 0} ∕ ξ_{a b 0}^{-}

, corresponding to the Ginzburg-Landau parameter

κ_{a b}

, decreases substantially and

Y {Ba}_{2} {Cu}_{3} O_{7 - δ}

crosses over from an extreme to a weak type-II superconductor.

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