Phototinia Red Robin ‘ball’ pruning

Hi - I have a tree ball of PRR and would like it to grow into a bigger ball, so have left it to sprout...I’m thinking it’s too cold here (Central Scotland) so will leave it Until spring .. but it’s spindly shoots so do I prune it back into a ball shape and hope that more shoots will grow and fill it out??
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https://forum.gardenersworld.com/discussion/1010515/how-to-change-your-username/p1