Various arylenediimides (ADIs) have been applied to construct photoresponsive coordination polymers (CPs), while the size effect of ADI π-conjugate systems on the photoresponsive behaviors in CPs has been overlooked in the past few years. Herein, we emphasize the size effect of ADI π-conjugate systems on photoinduced electron transfer (ET) in CPs, taking two Eu3+-based CPs, [Eu(H2BINDI)(BINDI)0.5(H2O)2]·NH2(CH3)2·8H2O (1) and [Eu2(BIPMDI)(DMF)4(NO3)2]·H2O·2DMF (2) [H4BINDI = N,N'-bis(5-isophthalic acid)naphthalenediimide; H4BIPMDI = N,N'-bis(5-isophthalic acid)pyromellitic diimide; DMF = N,N-dimethylformamide], as a case. Both 1 and 2 display ET-based photochromic behaviors with distinct photoresponsive rates and coloration contrast, which can contribute to the size effect of diimide cores on the interfacial contacts of electron donors/acceptors. Meanwhile, ET between the neighboring larger NDI cores of the H4BINDI ligands can block ligand-to-metal charge transfer and quench luminescence of the Eu3+ metal center in 1. Therefore, this work will provide a theoretical basis for the development and exploration of photoresponsive materials.