what is happening is that you're setting all font sizes to 12. I haven't figure out how to set just the xticks to 12 but you can reset the legend like the code below
figure1 = figure;
% Create axes
axes1 = axes('Parent',figure1);
box(axes1,'on');
hold(axes1,'all');
set(axes1,'xticklabel', strsplit(num2str(10:14),' '),'fontsize',12)
% Create multiple lines using matrix input to plot
plot1 = plot(randi(4,10));
% Create legend
legend1 = legend(axes1,'show');
set(legend1,'FontSize',8);
