  Lithium niobate (LN) is a promising material for future complex
photonic-electronic circuits, with wide applications in fields like
communications, sensing, quantum optics, and computation. LN took a great
stride toward compact photonic integrated circuits (PICs) with the development
of partially-etched LN on insulator (LNOI) waveguides. However, integration
density is still limited for future high-compact PICs due to the partial edge
nature of their waveguides. Here, we demonstrate a fully-etched LN PIC platform
which, for the first time, simultaneously achieves ultra-low propagation loss
and compact circuit size. The tightly-confined fully-etched LN waveguides with
smooth sidewalls allow us to bring the bending radius down to 20 $\mu$m
(corresponds to 1 THz FSR). We have achieved compact high-$Q$ microring
resonators with $Q/V$ of 7.1 $\times$ 10$^{4}$ $\mu$m$^{-3}$, almost one order
of magnitude larger than previous demonstrations. The statistical mean
propagation losses of our LN waveguides is 8.5 dB/m (corresponds to mean
$Q$-factor of 4.9 $\times$ 10$^{6}$) even with a small bending radius of 40
$\mu$m. Our compact and ultra-low-loss LN platform shows great potential in
future miniaturized multifunctional integration systems. As complementary
evidence to show the utility of our platform, we demonstrate soliton microcombs
with an ultra-high repetition rate of 500 GHz in LN.
