In short, because of modern technology and our careful selection of a capacitor
employing that technology that does do the job.
In more detail, a simple capacitor consists of a pair of conductive plates
separated by an insulator called a "dielectric." Two of the main parameters of a
capacitor are:

its "capacitance"  namely how much electrical charge does it store for
each Volt applied between its plates, and

its "breakdown voltage"  namely how many Volts can be applied between its
plates before it stops acting like a capacitor, for example because the
dielectric breaks down and a spark jumps between the plates and in all
likelihood damages the dielectric.
The capacitance is related to the area of the plates, multiplied by the
"dielectric constant" of the dielectric, and divided by the distance between the
plates.
The breakdown voltage is related to the "dielectric strength" of the dielectric
multiplied by the distance between the plates.
If you start with one design of capacitor and want to make it smaller, but with
the same capacitance and the same breakdown voltage, then:

Each time you halve the area of the plates, you need to halve the distance
between the plates, or double the dielectric constant of the dielectric (or a
combination of the two) so as to maintain the capacitance, BUT

Each time you halve the distance between the plates, you need to double
the dielectric strength of the dielectric so as to maintain the breakdown
voltage.
So, you need a dielectric whose dielectric constant multiplied by its dielectric
strength is higher.
The original Lucas capacitors has a dielectric of waximpregnated paper, whereas
BTH used mica. By contrast, the EasyCap capacitor has one of the many ceramics
that have been developed for use in capacitors today. A guide to the dielectric
constant and dielectric strength of these materials and air, together with the
allimportant product of the two, is:
Material 
Dielectric
constant 
Dielectric strength
(V/µm) 
Dielectric constant
x dielectric strength
(mid range) 
Air 
1 
3 
3 
Waxed paper 
4 
40  60 
200 
Mica 
5 
120 
600 
X7R ceramics 
1,000  4,000 
75  150 
280,000 
The discussion here is necessarily simplified, because this is a very
complicated subject. We are always ready, in fact we are keen, to enter into
technical discussion with those whose views may diverge from ours, or who can
otherwise add to the debate constructively.
We include the discussion because we all have a natural tendency to doubt
claims made for new products which are strikingly different from what they
replace, unless legitimate queries are addressed and explained. We have the same
tendency ourselves, in abundance.
In the days when Lucas and BTH and Bosch and others were designing their
magnetos, the sorts of materials used to make the capacitors used in EasyCaps
hadn't been invented. The constructors were therefore forced to use larger items
than they would have wished, to obtain satisfactory performance. This led to
sizeable condensers having to be buried deep in the armatures of all rotating
coil magnetos. Had the manufacturers had the option, they may well have ended up
mounting their condensers where they could be accessed easily as a routine
service item, which they were of course able to do with magnetos with rotating
magnets like the Lucas SR series, which are much loved by their owners for this
and for other reasons.
As an addendum, one reason why many owners regard the BTH rotating coil
magneto as superior to the Lucas equivalent is because of the use of mica as the
dielectric in the BTH condenser. As the table shows, it has superior dielectric
strength to waxed paper, and it also has higher resistance to humidity. Thus an
original BTH item is more likely to have survived in working order than a Lucas
item.
A very good paper on the volumetric efficiency of capacitors can be read by
clicking here.
