In order to understand the formation and evolution of small terrestrial planets not only their internal structure but also the architecture of their host system must be known. Systems harboring long-period cold gas and icy giants (a > 1 au) tend to host close-in small planets (a < 0.3 au, Rp < 4 R⊕) (Masuda et al. 2020). How does the presence of outer planets influence the formation and evolution of their inner smaller siblings? Unfortunately, our knowledge is still incomplete. Currently, the sample size for these systems is small and the relevant parameters are unknown. To truly understand the physics governing the formation and evolution of small planets we need to know the orbital architecture of their planetary systems. Here we present results from an ongoing radial velocity (RV) follow-up program conducted with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, mounted at the ESO 3.6m telescope of La Silla Observatory, and its twin for the Northern Hemisphere, the HARPS-N spectrograph, installed at the Telescopio Nazionale Galileo. Our program aims at measuring the mass of small transiting planets discovered by the TESS space telescope. The 4-year baseline of our project is allowing us to search for long-period outer planets, "expanding the horizon" and probing the architecture of planetary systems with inner small transiting planets.