Muonic atoms are many, many times smaller, and neutral — so small that the reduction in atomic radius by substituting a muon for an electron can catalyze fusion. Purely muon helium would be the smallest such. (Tauons can’t form atoms as their decay time is shorter by orders of magnitude than would be needed.)
You are right, so in order to say that the helium atom is the tiniest, the statement should be additionally qualified, by saying that it is the tiniest among the stable neutral atoms, in order to exclude the muonic atoms.
