I heartily endorse Chaitanya Kumar

SEM is touted as a non-destructive technique, but that is not always dtrue. Coccus bacteria might be considered to look like micro grapes in their hydrated states. Unless great care is taken to fix the material in place, they will quickly turn into micro raisins in the SEM as the water evaporates. This seems to be a similar situation.

As always, it would be nice to have more information when you say that "the sizes are different". What are the sizes and how narrow are the distributions? A scaled SEM image would be nice along with a DLS report/plot.

Another thing that should enter the discussion is the manner in which the average sizes are determined. This is especially a concern for wide distributions. If one distribution is calculated based on particle count while the other is based on particle mass, the exact same material will give two strikingly difference distributions. A few large particles will skew the mass distribution to the high side. Numerous small particles will skew the count distribution to the small side, but they may not amount to much of the sample mass. It is not that either method is wrong, but to compare them, they need to be on the same basis. The basis should also be chosen according to what is relevant to your research question.

DLS (Dynamic Light Scattering) and SEM (Scanning Electron Microscopy) measure particle size in fundamentally different ways, leading to differences in the reported values. DLS measures the hydrodynamic size of particles in a solution, which refers to the effective diameter of the particle as it moves through the solvent, influenced by factors such as particle shape, surface properties, and the surrounding solvent. It provides an average size of particles in the form of a hydrodynamic radius, which reflects the diffusion rate in Brownian motion. In contrast, SEM directly visualizes the dry, solid particle structure by scanning the surface morphology at high resolution, providing a static, surface-based measurement that is sensitive to shape, surface roughness, and aggregation, often leading to a smaller particle size compared to DLS. Thus, DLS tends to measure the "apparent" size in solution, while SEM reflects the actual particle size in a dry state.

I addition to the previous answers, take also into account that DLS analysis can be obtained in several size distribution modes (intensity, number, volume or mass, and area). For non-absolutely monomodal samples DLS results give different size distributions when they are expressed in the previous modes. All of them are correct but express different characteristics of the sample.

The usual way to express SEM results is in number (or frequency) distribution, so that, the modality of the sample as well as the type of distribution have to be taken into account when comparing both techniques.