There is a difference between quiver and streamslice that quiver returns a Quiver object while streamslice returns a vector of handles to the line objects. These line objects represent lines and arrows in the figure. Like any line object, the arrow properties can be modified to change Color, LineWidth, etc. Have a look at the sample code to understand better:
s = numel(streamslice(x,y,z,u,v,w,,,5,'noarrows'));
You can try replacing your streamslice function in the above code snippet and adjust LineWidth value to suit the size of arrowhead you need.