Your question is not detailed. I guess that the additional compound will be influence the "surface" of yor particles, resulting in a different stability of the dispersion. 

I see no correlation with effects of kinetic.

Article Functionalized polymer colloids bearing primary amino groups

Dear Dr. Schmitt,

Thank you so much for your answer.

I scanned your paper. What you have done is indeed emulsion polymerization whose mechanism is different from that of dispersion polymerization.

Moreover, the comonomers you have used in your system are hydrophilic. One is anionic, another is cationic. So, irrespective of their role on kinetic, they heavily affect thermodynamic aspects of the system. They tend to migrate toward particle surface. They first form nuclei inside which the reaction of BuA proceeds. Overall, they change everything.

My system is different, where by adding a monomer (similar in terms of polarity, and hydrophobicity , but different in terms of propagation constant of polymerization), the results is improved remarkably....

Let me elaborate on my system. Let's consider the main monomer is 2-ethyl hexyl acrylate (2-EHA)

Poly(vinyl pyrolidone) as stablizer, water/ethanol as medium, Azobis isobutyronitrile (AIBN) as initiator.

Now, when I add a small amount of butyl acrylate (BuA) to the system, particle size is decreased.

 warm regards, and thanks for your consoderation

Maybe reading the book below may help you. It is from ICI people, who did a lot on this techniques in the 60-ies, 70-ies in their corporate lab.

Next combine a search on who is referring to this book for the question you have by doing a literature search. One can imagine that using e.g. butyl acrylate compared to MMA, may have a large effect on the seeding stage and the rest of the growth, also depending upon the solvent mix you use!. Dispersion polymerization is different from emulsion polymerization wrt its kinetics. Realize the difference in the locus of the polymerization et al. The article from Michael Schmitt shows a nice latest case. 

Dispersion Polymerization in Organic Media

 Keith E. J. Barrett

Wiley, 1974 - Polymerization - 322 pages

not an expert in polymerization I see a marked difference in ethyl hexyl and in butyl hydrophobic group - the latter having smaller size (maybe diffuses faster) and is not branched. Beside this there might be a synergistic effect regarding packing.

You're doing the process as a one shot batch process, no dosing of monomers?

To get an idea what is (especially) at the seeding stage and short after that: 

Quench (HQ or MeHQ) at intervals, also at the very start, samples taken out of the reactor and determine monomer conversion of both. Then you may plot the conversion of both at any time. Suppose BA is converted much faster (than expected or than 2EHA) this fast conversion may impact the nucleation stage; smaller seeds may lead to smaller end particles.  I do not have at this moment the r1 and r2 values to see how they copolymerize, but I would normally not expect very large differences. So that's why the conversion check in time is need to check deviations from expectations...

Faster initial polymerization/ seed formation will have certainly influence the end particle size. 

As both are apolar and have a low polymer Tg, I do not expect effects from these factors... and if.. I'm very curious to learn on it.

you can revert to the book of principales of polymerization

you will find your answer