Mesoporous polymer nanoparticles have broad applications in drug delivery, bioengineering, and catalysis. Selective swelling of block copolymer (BCP) is one promising strategy to generate controllable and switchable porous polymer nanostructures. Previous studies of selective swelling of block copolymer nanostructures revealed that the swelling condition, BCP composition, their architectures and sizes are involved in structural transformation in swelling. To clearly understand the effect of these factors, herein we compared the swelling behaviors of two types BCP nanoparticles, large compound micelles, and onion-like particles, with different sizes and compositions. The composition of block copolymers, the nanostructure of block copolymer nanoparticles, and the size of block copolymer nanoparticles are investigated separately to understand how these factors intertwine in determining swelling speed, structural transformation in swelling process, and the final swollen nanostructures. This study helps us understand how to elegantly control the porous structure generated by selective swelling of block copolymers.