Quantitative Ultrasound techniques for determining the tissue microstructure rely on theoretical scattering models to fit the BackScatter Coefficient (BSC) from biological tissues to an estimated BSC using an appropriate model. The models adapted to densely packed cells in tumors are the Structure Factor model (SFM) and the Particle Model (PM, i.e. the low-frequency limit of the SFM), which are generally used in blood characterization. The aim of this work was to use the SFM and the PM to go further in the understanding of the measured BSCs from simple tumor models (i.e. cell pellet biophantoms). Ultrasonic backscatter measurements were performed at frequencies ranging from 10 to 42 MHz on biophantoms. These biophantoms consisted of human leukemia K562 cells trapped in a mixture of plasma and thrombin with different cell concentrations ranging from 0.006 to 0.30. A parameter estimation procedure was developed in order to estimate the scatterer size and relative impedance contrast that could explain the measured BSC from all the studied cell concentrations using three scattering models: the SFM, the PM and the classical Fluid-Filled Sphere Model (FFSM) (that does not account for dense medium) for polydisperse scatterer size distribution. This procedure was applied to our BSC data from K562 cell pellet biophantoms in the 10-42 MHz frequency bandwidth and to the BSC data from Chinese Hamster Ovary (CHO) cell pellet biophantoms in the 26-105 MHz frequency bandwidth given in [Han et al., J. Acoust. Soc. Am. 130, 2011]. The scatterer sizes estimated using our parameter estimation procedure were compared to the true cellular features to identify the scattering sites. The results obtained with the K562 cells (in the 10-42 MHz frequency bandwidth) and with the CHO cells (in the 26-56 MHz frequency bandwidth) revealed that the FFSM and PM are insufficient to model the complex behavior of the BSC and that the polydisperse SFM is the model that better explains the behavior of BSC. The impedance and size estimated with the polydisperse SFM are satisfactory: the relative impedance contrast estimates seem to be in a reasonable range of values and the fluid sphere radii match the true whole cell radii for both K562 and CHO cell studies. This finding shows that the whole cell plays a major role in the BSC behavior for the K562 and CHO cells studied.