The described issue is very strange and we have not encounter it so far and we are not sure what could be the reason for it. I tried to reproduce it on my side, but to no avail. Could you please post your aspx code with the related code behind file. Thus we will be able to gather more details about your scenario and provide you with more to-the-point answer.
At a meantime could you please verify that you do not have any css style with force the slider to be such small?
Looking forward for your reply.
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