A quick look on wikipedia (https://en.wikipedia.org/wiki/W_and_Z_bosons) will tell me that the rest mass of a $W$ boson is 2 orders of magnitude higher than that of a proton.
It seems weird that a particle would 'spit out' something 100x more massive than it - is it simply because weak interactions will only occur when a proton or neutron is at very high energy (so has a lot to get rid of as mass)?
Copied from https://www.open.edu/openlearn/science-maths-technology/particle-physics/content-section-8.1 (A W− boson is emitted with one unit of negative electric charge, so conserving electric charge in the process. The mass energy of the W− boson is about 80 GeV, so it cannot possibly emerge from the nucleus as there are only a few MeV of energy available. In accordance with the energy–time uncertainty principle it therefore rapidly decays to produce an electron and an electron antineutrino, setting the energy accounts straight. ) This is a little misleading as it suggests that the w-boson is actually 80Gev rather than 0.002 Gev which it is able to be BECAUSE of the energy-time uncertainty principle. So to clarify see also this (https://youtu.be/yOiABZM7wTU) regarding the probability of this and explaining why the weak force is weak. (From 6m50s).
The proton/neutron can emit a W boson that has more mass than itself because
A)the W boson can be a special W boson which is actually quite light. This W boson is heavy enough to turn into an electron/positron and neutrino/antineutrino in radioactive decay.
B)the W boson is a virtual particle that just borrows mass from the vacuum allowing for the conservation of energy to be violated for a split second-enough to allow radioactive decay- and allow these particles to be produced.
