Lattice contraction introduced by the smaller covalent radius of carbon (77 pm) relative to arsenic (120 pm), gallium (126 pm), and indium (144 pm) has been observed in highly carbon doped GaAs. This paper addresses similar effects in heavily doped InGaAs:C layers, which find applications as the base of InP/InGaAs heterojunction bipolar transistors (HBTs). Heavily carbon doped InGaAs epilayers (p>1×1019cm-3) have been grown by metalorganic chemical vapor deposition (MOCVD) using CBr4 as the carbon precursor. The lattice contraction effect induced by carbon incorporation has been studied in such samples by extending the technique previously reported for heavily doped GaAs:C. High-resolution double crystal x-ray diffraction (DXRD) as well as Hall measurements have been performed for this purpose. The experiments clarify the way lattice contraction takes place in case of strong carbon incorporation in InGaAs:C samples. They also show that InGaAs:C becomes increasingly compensated as the doping concentration increases. The study has direct impact on the development of high quality InP-based HBTs.