Rectangle function produces blurry circles

Hi all, I'm making a circle filling program, and for some reason, about half of the circles produced don't have crisp borders like I want. For some reason, they're almost kind of blurry or fuzzy. How can I get rid of this behavior?

clear
clc

WIDTH   = 800;
HEIGHT  = 400;
testMatrix = [1, 1, 0, 0; 0, 0, 1, 1; 1, -1, 1, -1; 0, 0, 0, 0];

window = figure('Toolbar', 'none', 'Menubar', 'none',...
                'Position',[300,300,WIDTH,HEIGHT], 'Resize', 'off',...
                'Name', 'Circle Filling');
hold on
ax = axes('Position', [0,0,1,1], 'box', 'off');

% each circle = [x, y, r, growth, r, g, b]
circles = [round(WIDTH*rand()), round(HEIGHT*rand()), 1, 1, rand(),rand(),rand()]; 

growingCircles = circles(:,4) ~= 0; % mask of all circles that are currently growing

for frame = 1:250

      circlesToDraw = circles(growingCircles, :);  % Draw growing circles
      for index = 1:size(circlesToDraw)
          circle = circlesToDraw(index,:);
          rectangle(ax,'Position',[circle(1:2)-circle(3), 2*circle(3), 2*circle(3)],...
                   'Curvature',1,'EdgeColor', circle(4:7),'FaceColor', circle(4:7))
      end

      ax.Color = [0,0,0];
      axis([0,WIDTH,0,HEIGHT]);

      indexCircles = 1:size(circles);
      for circle =  indexCircles(growingCircles) % test if growing circles need to stop growing
          bounds = circles(circle,1:4)*testMatrix;
          if sum(bounds < 0 | [WIDTH < bounds(1:2), HEIGHT < bounds(3:4)]) ~= 0
              circles(circle,4) = 0;
              continue
          end

          for testAgainst = 1:size(circles)
              if circle ~= testAgainst
                  rSum = sum(circles([circle, testAgainst],3));
                  distance = sqrt(sum((circles(circle,1:2)-circles(testAgainst,1:2)).^2));
                  if distance < rSum +1
                      circles([circle, testAgainst],4) = 0;
                  end
              end
          end
      end

      growingCircles = circles(:,4) ~= 0; % Update growingCircles

      circles(growingCircles,3) = circles(growingCircles,3) + circles(growingCircles,4); % Update Circles

      % Attempt to create new circle
      for index = 1:1
          newPos = [round(WIDTH*rand()), round(HEIGHT*rand())];
          dist = sqrt(sum(((circles(:,1:2)-newPos).^2), 2)); % Get a column vector of newPos distance to each circle
          if sum(dist < circles(:,3)) == 0 
              circles(end+1,:) = [newPos, 1, 1, floor(10*[rand(),1-rand(),rand()])/10];
              growingCircles = [growingCircles; true];
          end
      end

      clc
      fprintf('Frame: %d', frame);
      pause(.001)
  end