[GAP Forum] Unexpected Behaviour in Size()

[Nilo de Roock]

Hello GAP forum,

I created the free group ( i got this spec. for the group from a  
textbook-exercise ):
gap> F:=FreeGroup(3);
<free group on the generators [ f1, f2, f3 ]>
gap> x:=F.1;
f1
gap> y:=F.2;
f2
gap> z:=F.3;
f3
gap> G:=F/[y^3*z^15,x^4*y^7*z^3,x^8,y^14,z^18];
<fp group on the generators [ f1, f2, f3 ]>

Then when I wanted to now the size of the group, GAP became a bit of  
erratic.

gap> Size(G);
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
user interrupt at
firstFree := app[6];
  called from
TCENUM.CosetTableFromGensAndRels( fgens, grels, fsgens ) called from
CosetTableFromGensAndRels( fgens, grels, List( trial, UnderlyingElement )
  ) called from
Attempt( gens ) called from
FinIndexCyclicSubgroupGenerator( G, infinity ) called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> quit;
#I  Options stack has been reset
gap> G:=F/[y^3*z^15,x^4*y^7*z^3,x^8*y^14*z^18];
<fp group on the generators [ f1, f2, f3 ]>
gap> Size(G);
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
#I  Coset table calculation failed -- trying with bigger table limit
exceeded the permitted memory (`-o' command line option) at
g[2 * limit] := 0;
  called from
TCENUM.CosetTableFromGensAndRels( fgens, grels, fsgens ) called from
CosetTableFromGensAndRels( fgens, grels, List( trial, UnderlyingElement )
  ) called from
Attempt( trial ) called from
Attempt( gens ) called from
FinIndexCyclicSubgroupGenerator( G, infinity ) called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

Sorry but, any idea what's wrong in this case? Thanks on beforehand for  
any advice.

-- 
met vriendelijke groet,
Nilo de Roock