Improper ferroelectrics are highly promising for technological applications due to their expected persisting polarizations even at ultrathin limits. However, the evolution of their electrical behaviors with thickness, including the magnitude of the polarization and the switching mechanism, remain unresolved experimentally. This is primarily due to the difficulty in growth and characterization of ultrathin improper ferroelectric films. Here, we investigated the spontaneous polarization and switching mechanism in ferroelectric/dielectric bilayer structures, which allows the decoupling of the electrostatic and interfacial effects and circumventing the need for ultrathin films. We show that, for the bilayer structures of prototypical improper ferroelectrics h-YbFeO3 and dielectrics CoFe2O4, although the effective spontaneous polarization is significantly reduced by the dielectric layer due to the electrostatic under-screening, it persists at least down to a ferroelectric/dielectric thickness ratio of about 2, with no evidence of critical thickness. Interfacial clamping that suppresses the primary structural distortion of h-YbFeO3 have been observed, which appears to play an important role in ferroelectric domain pinning. The microstructure caused by the heteroepitaxy favors a nucleation-limited polarization switching dynamics. These results demonstrate the much-desired absence of critical thickness in improper ferroelectrics for the scalable thin-film device applications; they also reveal significant impact of the mismatched film/substrate epitaxy on the polarization switching mechanism.